Allow complete override of a Session's Headers

[prkumar]

Is your feature request related to a problem? Please describe. It would be nice if I could set the entire session.headers and not just update individual keys.

Describe the solution you'd like I want to be able to override the session.headers property entirely:

self.session.headers = other_consumer_class.headers.copy()

This would require adding a setter for the headers property on the Session class:

class Session(object):
    ...
    @headers.setter
    def headers(self, headers):
        ...