Content-Encoding is about gzip, etc. Not character encoding.

See github issues #1872, #961 and http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html for more info.
Add assertions to prevent reintroducing this bug in the future.
Merge pull request #1982 from chrisvest/remove-content-encoding-headers

Content-Encoding is about gzip, etc. Not character encoding.