ContentType: Add __str__ and __repr__.
ContentType: Add __eq__.
parse_content_type: Add RFC 7231 examples.

Forced me to address the fact that content type parameter values
are listed as case-insensitive in section 3.1.1.1 of RFC 7231.
As noted in the code, RFC 2045 mentions that there are cases where
parameter values are case preserving.  A new keyword parameter was
added to address this.
ContentType: Strip type/subtype values.

This was exposed whilst testing accept header parsing.
ContentType: Implement ordering.
ietf.headers: Add parse_accept_header.