fix#325
This removes unused and misleading handshake error codes.
All semantic handshake failures are now reported using
the same code, error::handshake_failed. Errors originating
from stream operations still use the underlying stream
error codes (for example: boost::asio::error::connection_reset).
Core:
* Test buffer_cat iterator move members
HTTP:
* Fixed yield / resume in writer
* Fixed message serialization with chunked encoding
* Test yield / resume in writer
* Test all conditional branches during message serialization
* Test chunked encoding
* Increase coverage on parse_error
* Add parse_error::general
WebSocket:
* Add error::general
* Increase coverage in error
