text: Title: Allow complete override of a Session's Headers
  Body: **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`](https://github.com/prkumar/uplink/blob/master/uplink/session.py) class:
  
  ```python
  class Session(object):
      ...
      @headers.setter
      def headers(self, headers):
          ...
  ```