Support for .xhtml

I’ve attempted to make support for .htm and .xhtml consistent.
Extensions are in alphabetical order to make it easier to compare them.

middleman-core/features/gzip.feature

@@ -22,7 +22,7 @@ Feature: GZIP assets during build
     Given a fixture app "gzip-app"
     And a file named "config.rb" with:
       """
-      activate :gzip, exts: %w(.js .html .htm)
+      activate :gzip, exts: %w(.htm .html .js .xhtml .xml)
       """
     And a successfully built app at "gzip-app"
     Then the following files should exist:

middleman-core/lib/middleman-core/application.rb

@@ -154,7 +154,7 @@ module Middleman
 
     # Which file extensions have a layout by default.
     # @return [Array.<String>]
-    define_setting :extensions_with_layout, %w(.html .xhtml .php), 'Which file extensions have a layout by default.'
+    define_setting :extensions_with_layout, %w(.htm .html .xhtml .php), 'Which file extensions have a layout by default.'
 
     # Default string encoding for templates and output.
     # @return [String]

middleman-core/lib/middleman-core/extensions/asset_hash.rb

@@ -2,7 +2,7 @@ require 'middleman-core/util'
 require 'middleman-core/rack'
 
 class Middleman::Extensions::AssetHash < ::Middleman::Extension
-  option :sources, %w(.htm .html .php .css .js), 'List of extensions that are searched for hashable assets.'
+  option :sources, %w(.css .htm .html .js .php .xhtml), 'List of extensions that are searched for hashable assets.'
   option :exts, %w(.jpg .jpeg .png .gif .webp .js .css .otf .woff .woff2 .eot .ttf .svg .svgz), 'List of extensions that get asset hashes appended to them.'
   option :ignore, [], 'Regexes of filenames to skip adding asset hashes to'
   option :rewrite_ignore, [], 'Regexes of filenames to skip processing for path rewrites'

middleman-core/lib/middleman-core/extensions/asset_host.rb

@@ -3,7 +3,7 @@ require 'addressable/uri'
 class Middleman::Extensions::AssetHost < ::Middleman::Extension
   option :host, nil, 'The asset host to use or a Proc to determine asset host', required: true
   option :exts, %w(.css .png .jpg .jpeg .webp .svg .svgz .js .gif), 'List of extensions that get cache busters strings appended to them.'
-  option :sources, %w(.htm .html .php .css .js), 'List of extensions that are searched for bustable assets.'
+  option :sources, %w(.css .htm .html .js .php .xhtml), 'List of extensions that are searched for bustable assets.'
   option :ignore, [], 'Regexes of filenames to skip adding query strings to'
   option :rewrite_ignore, [], 'Regexes of filenames to skip processing for host rewrites'
 

middleman-core/lib/middleman-core/extensions/cache_buster.rb

@@ -1,7 +1,7 @@
 # The Cache Buster extension
 class Middleman::Extensions::CacheBuster < ::Middleman::Extension
   option :exts, %w(.css .png .jpg .jpeg .webp .svg .svgz .js .gif), 'List of extensions that get cache busters strings appended to them.'
-  option :sources, %w(.htm .html .php .css .js), 'List of extensions that are searched for bustable assets.'
+  option :sources, %w(.css .htm .html .js .php .xhtml), 'List of extensions that are searched for bustable assets.'
   option :ignore, [], 'Regexes of filenames to skip adding query strings to'
   option :rewrite_ignore, [], 'Regexes of filenames to skip processing for path rewrites'
 

middleman-core/lib/middleman-core/extensions/gzip.rb

@@ -7,10 +7,10 @@
 # to serve your Gzipped files whenever the normal (non-.gz) filename is requested.
 #
 # Pass the :exts options to customize which file extensions get zipped (defaults
-# to .html, .htm, .js and .css.
+# to .css, .htm, .html, .js, and .xhtml
 #
 class Middleman::Extensions::Gzip < ::Middleman::Extension
-  option :exts, %w(.js .css .html .htm .svg), 'File extensions to Gzip when building.'
+  option :exts, %w(.css .htm .html .js .svg .xhtml .xml), 'File extensions to Gzip when building.'
   option :ignore, [], 'Patterns to avoid gzipping'
   option :overwrite, false, 'Overwrite original files instead of adding .gz extension.'
 

middleman-core/lib/middleman-core/extensions/relative_assets.rb

@@ -3,7 +3,7 @@ require 'addressable/uri'
 # Relative Assets extension
 class Middleman::Extensions::RelativeAssets < ::Middleman::Extension
   option :exts, %w(.css .png .jpg .jpeg .webp .svg .svgz .js .gif .ttf .otf .woff .woff2 .eot), 'List of extensions that get cache busters strings appended to them.'
-  option :sources, %w(.htm .html .css), 'List of extensions that are searched for relative assets.'
+  option :sources, %w(.css .htm .html .xhtml), 'List of extensions that are searched for relative assets.'
   option :ignore, [], 'Regexes of filenames to skip adding query strings to'
   option :rewrite_ignore, [], 'Regexes of filenames to skip processing for path rewrites'