Use configurable content type for detection of minifiable content.

Squashed changes:

  - Prevent side effects of content type testing.
  - Test for inline minification in PHP files.

middleman-core/fixtures/minify-css-app/source/inline-css.php

@@ -0,0 +1,8 @@
+<?='Hello'?>
+
+<style>
+  body {
+    test: style;
+    good: deal;
+  }
+</style>

middleman-core/fixtures/minify-css-app/source/stylesheets/site.xcss.sass

@@ -0,0 +1,5 @@
+@import "compass/reset"
+
+@media handheld, only screen and (device-width: 768px)
+  body
+    display: block