Add cow_http_hd:parse_te/1

From RFC7230.
Fix a variable shadow issue in the tests
Add dummy functions for Sec-WebSocket-(Accept|Key) headers

From RFC6455.

These base64 values are expected to be matched directly in the
first case, or hashed to generate a new base64 value, therefore
no parsing or validation is required.
Use _req instead of _client for request-specific parsers
Add cow_http_hd:parse_sec_websocket_protocol_resp/1

From RFC6455.