Don't destructively modify SafeBuffers.

Closes gh-399
Closes gh-400

doc-src/HAML_CHANGELOG.md

@@ -12,6 +12,8 @@
 
 * Fix html2haml.
 
+* Fix an issue where destructive modification was sometimes performed on Rails SafeBuffers.
+
 ## 3.1.1
 
 * Update the vendored Sass to version 3.1.0.

lib/haml/compiler.rb

@@ -379,7 +379,7 @@ END
         value = Haml::Helpers.preserve(escaped)
         if escape_attrs
           # We want to decide whether or not to escape quotes
-          value.gsub!('"', '"')
+          value = value.gsub('"', '"')
           this_attr_wrapper = attr_wrapper
           if value.include? attr_wrapper
             if value.include? other_quote_char

lib/haml/helpers.rb

@@ -170,10 +170,10 @@ MESSAGE
         result = capture_haml(i, &block)
 
         if result.count("\n") > 1
-          result.gsub!("\n", "\n  ")
+          result = result.gsub("\n", "\n  ")
           result = "\n  #{result.strip}\n"
         else
-          result.strip!
+          result = result.strip
         end
 
         "<li>#{result}</li>"