Add cow_http_hd:parse_expect/1

From RFC7231.
Optimize cow_http:parse_fullhost/1

Using two functions instead of an argument is a much better way
to handle two specific pathways in the code.
Add cow_http_hd:parse_max_forwards/1

From RFC7231.
Add cow_http_hd:parse_accept/1

From RFC7231.

This code is more than twice faster as the current Cowboy code,
while filtering out more bad cases.
Add cow_http_hd:parse_charset/1

From RFC7231.

This code is more than twice faster as the current Cowboy code,
while filtering out more bad cases.
Add cow_http_hd:parse_accept_encoding/1

From RFC7231.
Add cow_http_hd:parse_accept_language/1

From RFC7231.

This code is more than twice faster as the current Cowboy code,
while filtering out more bad cases.
Add testing for bad input for cow_http_hd:parse_content_length/1