Make sure that the Encoding check does not modify the given string

this partially reverts 6ce26c83ad
see: 6ce26c83ad (commitcomment-2586688)

lib/haml/util.rb

@@ -103,8 +103,7 @@ module Haml
     # @return [String] `str`, potentially with encoding gotchas like BOMs removed
     if RUBY_VERSION < "1.9"
       def check_encoding(str)
-        str.gsub!(/\A\xEF\xBB\xBF/, '') # Get rid of the UTF-8 BOM
-        str
+        str.gsub(/\A\xEF\xBB\xBF/, '') # Get rid of the UTF-8 BOM
       end
     else
 
@@ -113,11 +112,12 @@ module Haml
           # Get rid of the Unicode BOM if possible
           # Shortcut for UTF-8 which might be the majority case
           if str.encoding == Encoding::UTF_8
-            str.gsub!(/\A\uFEFF/, '')
+            return str.gsub(/\A\uFEFF/, '')
           elsif str.encoding.name =~ /^UTF-(16|32)(BE|LE)?$/
-            str.gsub!(Regexp.new("\\A\uFEFF".encode(str.encoding)), '')
+            return str.gsub(Regexp.new("\\A\uFEFF".encode(str.encoding)), '')
+          else
+            return str
           end
-          return str
         end
 
         encoding = str.encoding

test/templates/with_bom.haml

@@ -0,0 +1 @@
+BOMG

test/util_test.rb

@@ -60,4 +60,15 @@ RUBY
       c.send(static_method_name(:static_method, false, false),
         "brush your teeth", "play with fire"))
   end
+
+  def test_check_encoding_does_not_destoy_the_given_string
+    string_with_bom = if RUBY_VERSION > '1.9'
+      File.read(File.dirname(__FILE__) + '/templates/with_bom.haml', :encoding => Encoding::UTF_8)
+    else
+      File.read(File.dirname(__FILE__) + '/templates/with_bom.haml')
+    end
+    original = string_with_bom.dup
+    check_encoding(string_with_bom)
+    assert_equal(original, string_with_bom)
+  end
 end