Fix payload length handling (RFC 6455, section 5.2)

Fixes Autobahn Testsuite websockets case 1.1.2
Use readexactly method, because payload maybe choped up

Fixes Autobahn Testsuite websockets cases 1.1.8, 1.2.8
Merge pull request #1 from etataurov/fixrfc

AutobahnTestsuite