'Add Basic Authentication header to all the requests.
It injects the \'Authorization: Basic Base64String===\' header
in each request
:type  headers: ``dict``
:param headers: Default input headers
:rtype:         ``dict``
:return:        Default input headers with the \'Authorization\'
header'

def add_default_headers(self, headers):

    b64string = b(('%s:%s' % (self.user_id, self.key)))
    encoded = base64.b64encode(b64string).decode('utf-8')
    authorization = ('Basic    ' + encoded)
    headers['Authorization'] = authorization
    return headers
