Allow the clients to decide if validation of SSL certificates should be skipped

Instead of just assuming that in DEBUG root certificates should not be evaluated allow the callers to decide.
In order to not break existing clients, in DEBUG evaluation still is off, but in release builds you can set the allowUntrustedSSLCertificates flag.
Because it is the Square iOS style; all our code looks like that. The rationale is that it is convenient for copy/pasting methods between header and implementation files.