POST binding not passed through

[Jan Vilhuber]

Just noticed something else:

https://github.com/rohe/pysaml2/blob/master/src/saml2/client.py#L388

Shouldn't binding be passed in here? If I use an HTTP POST binding,
and I pass this into the caller, it is not passed in HERE, and thus we
look for an HTTP REDIRECT binding (which my particular metadata I'm
testing with doesn't have).

i.e.

        location = self._sso_location(entityid)
should be

        location = self._sso_location(entityid, binding)