Added support for content encoding.

Added test for gzip/deflate resources with the "Content-Encoding" header.  Added check for content encoding header in addition to transfer encoding to support gzip/deflate resources.

I based my changes on real-world needs and the following Wikipedia articles.

[HTTP Compression](http://en.wikipedia.org/wiki/HTTP_compression)
[List of HTTP header fields](http://en.wikipedia.org/wiki/List_of_HTTP_header_fields)