Make swizzled URLSession init method signature match the origin init method to avoid unexpected crash

[ivanlisovyi]

Hi @kean, first of all, thank you for creating such a cool library!

After the integration using the swizzling approach, we've noticed that our app started to crash occasionally. Apparently, the crash reason was related to the fact that pust_init URLSession method signature didn't really match the original init method. The swizzled method had delegate defined as non-optional whereas the original method has it defined as optional. Since we're swizzling the init method for every instance of URLSession created after a call of URLSession.enableAutomaticRegistration() it may lead to a crash when nil passed as a delegate value.

This PR shall also fix #29

[kean]

Thanks, @ivanlisovyi! I appreciate you taking your taking and fixing it.