Make an effort to handle spaces in filenames in a way that url_for can handle.

middleman-core/features/directory_index.feature

@@ -35,7 +35,7 @@ Feature: Directory Index
     
   Scenario: Preview normal file with spaces in filename
     Given the Server is running at "indexable-app"
-    When I go to "/evil spaces/"
+    When I go to "/evil%20spaces/"
     Then I should see "Spaces"
 
   Scenario: Preview normal file subdirectory

middleman-core/lib/middleman-core/cli/build.rb

@@ -119,7 +119,7 @@ module Middleman::Cli
       # @return [String] The full path of the file that was written
       def render_to_file(resource)
         build_dir = self.class.shared_instance.config[:build_dir]
-        output_file = File.join(build_dir, resource.destination_path)
+        output_file = File.join(build_dir, resource.destination_path.gsub('%20', ' '))
 
         if resource.binary?
           if !File.exists?(output_file)

middleman-core/lib/middleman-core/sitemap/resource.rb

@@ -38,7 +38,7 @@ module Middleman
       def initialize(store, path, source_file=nil)
         @store       = store
         @app         = @store.app
-        @path        = path
+        @path        = path.gsub(' ', '%20') # handle spaces in filenames
         @source_file = source_file
 
         @destination_paths = [@path]

middleman-core/lib/middleman-core/util.rb

@@ -153,6 +153,7 @@ module Middleman
     def self.url_for(app, path_or_resource, options={})
       # Handle Resources and other things which define their own url method
       url = path_or_resource.respond_to?(:url) ? path_or_resource.url : path_or_resource
+      url = url.gsub(' ', '%20')
 
       begin
         uri = URI(url)