Prevent a stack overflow due to recursion in GetPooledConnector

There's a chance that the recursion in the GetPooledConnector method can cause a stack overflow exception.
While this is unlikely, assumptions about how short the stack is, shouldn't be made.
ASP.NET MVC for instance already has a tall stack even before actions are called.

This change eliminates the possibility of overflowing the stack when GetPooledConnector is called.