Fix #76 Do not add a receipt handler to PING Frames, as this will cause a memory leak.

[DevMcC]

76 PING Frames would normally include a receipt handler, which never get resolved, so you will end up with an ever-increasing list of pending receipts.

Passing null as the receipt handler should prevent the creation of a pending receipt.

[vietj]

thanks, but that does not fix the issue that an user might send a PING frame on purpose and create leak the receipt handler. I created two PR's to fix that at the send level.