[Fixes 1428] Setting ContentEncoding header in ContentResult

@kichalla 
This is an incorrect header as `Content-Encoding` is used for compression/decompression in association with request's `Accept-Encoding` header...in the current case you would need to set the `CharSet` parameter on the `Content-Type` header...you can take a look a HTTP spec for more information regarding these headers...
http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html
Also please add some tests to verify this change... 

:watch: