Remove cowboy_http:urldecode/1 and urlencode/1

Use cow_qs:urldecode/1 and cow_qs:urlencode/1 instead
Remove R16B03-0 from the list of tested releases
Update cowlib to 0.6.0
Update erlang.mk
Use cow_http_hd:parse_connection/1 where applicable
Use cow_http_hd:parse_content_length/1 where applicable
Use cow_http_hd:parse_transfer_encoding/1 where applicable
Use the new chunked and identity transfer decoding code from cowlib
Remove cowboy_client; use gun for the HTTP test suite
Don't flush the resp_sent message if connection is closed

Tiny optimization.
Make the latin1 cyrillic route tests work on R17+

Instead of relying on the encoding of the file we now simply
have list of numbers as they would be inside a latin1 file.
Simplify the SPDY suite using the new Gun interface