* Initialize headers variable in BuildApiMixin.build() as a dict rather
  than as None.  This way the correct object gets passed to
  _set_auth_headers() even if no headers were set in build()

* Changing object from None to {} in BuildApiMixin._set_auth_headers()
  removed because it changes the object reference, so has no effect on
  calling code.
Signed-off-by: Justin Michalicek <jmichalicek@gmail.com>