fix: allow all frame types except 'reserved' to include payload

[yanshay]

All frame types except the 'reserved' opcodes should support payload, and therefore payload_len should not be zero.
Currently for Ping/Pong/Close payload_len is set to zero even if provided by the other party.
In each of them this is required and even useful.

• Ping - useful to measure for example roudtrip time,
• Close - provide reason for close.
• Pong - must return the payload sent by the ping.

I fixed for the client, since that's what I'm using.
I don't know if the same code is used by the server side.

[ivmarkov]

@yanshay I'm sorry! I actually forgot about this PR of yours (was meant to ask you to just update the changelog as well).

Anyway, I merged #67 which is anyway an exact copy of your PR. Thanks and sorry again for the ommission!