Remove invalid proxy behavior of Http and RetryTransport

[abergmeier]

Unless we make a proper proxy out of Http and RetryTransport, this generates a messed up object because one part of function calls go to
self and others delegate to _transport.

So only expose methods, which really delegate.

[dekkagaijin]

We don't want to give up the type, but it looks like we want to use the cert functionality, so I'll make the changes. In retrospect, the retry transport's factory should have taken a callable for underlying transports.

I think that there aren't so many implementations that we can't proxy calls to underlying transports for add_credentials, add_certificate, clear_credentials, and request. I'll make the internal change for that.

[abergmeier]

We don't want to give up the type

No problem. But then please make it a real proxy.