Only wrap attachment filenames in double quotes not other params

The current implementation is sticking quotes in odd places:

````
1.9.3p194 :018 > Mail::ContentTypeField.new.send(:ensure_filename_quoted, "audio/x-midi;\r\n\sname=Part .exe")
 => "\"audio/x-midi;\r\"\n name=\"Part .exe\""
````

After the fix:
````
1.9.3p194 :002 > Mail::ContentTypeField.new.send(:ensure_filename_quoted, "audio/x-midi;\r\n\sname=Part .exe")
 => "audio/x-midi;\r\n name=\"Part .exe\"" 
````

Not super thrilled with the nasty regexp, open to other ways of solving this problem.