GCDAsyncUdpSocket: fixed wrong condition for all methods that can be used in performBlock:

Fixes #211
fixed multicast operations for IPv6
Merge pull request #214 from MeckiCologne/master

fixed multicast operations for IPv6
Podspec updated to 7.3.4
GCDAsyncSocket: fixing -Wreceiver-is-weak warnings.

Fixes #206
Failing unit test to expose available space bugs in GCDAsyncSocketPreBuffer.
Fix GCDAsyncSocketPreBuffer availableSpace bugs that lead to access violation.

It's easy to see the bug by putting the read pointer slightly behind the
write pointer both near the end of the buffer.