Enforce case restrictions (lowercase) in Client.default_headers
Enforce case restrictions (lowercase) in Client.default_headers

Since we're already mutating self.default_headers from Client.__init__,
we ought to enforce the case restrictions we rely on. This is especially
important since the new API flexibility (à la ConnectionProperties)
will soon allow user-code to add support for their own APIs.

mozilla/agithub,jpaugh/agithub