Fixing fragmented WebSocket messages

Frames with a payload length of 127 would cause WSDecoder to throw an
exception after going from PAYLOAD to FRAME_START as the opcode would
not match. The fix was to set the variable "opcode" to OPCODE_CONT
before returning to state FRAME_START. Another exception would be thrown
on reading the final frame due to the first frame's opcode not being
preserved. The fix was to add the variable "fragmentedOpCode" that would
preserve the first frame's opcode. The changes should be in accordance
with Section 5.4 of RFC 6455.

The ProtocolException in READ_8_LENGTH had a misleading statement. For
example, the max payload length can be greater than 4M. It now provides
a more informative message.