Fixes all tests failing because of None secret passed to hmac.new introduced in 79c41e1

  - All tests were previously failing with error `TypeError: object of `NoneType` has no len()`:
    https://travis-ci.org/Instagram/python-instagram/jobs/58567079
  - If we weren't given a `client_secret`, we don't have a secret to sign the request with, so `_signed_request()`
    should just return the empty string.

Author: dtran320

instagram/oauth2.py

@@ -124,7 +124,7 @@ def _generate_sig(self, endpoint, params, secret):
         sig = endpoint
         for key in sorted(params.keys()):
             sig += '|%s=%s' % (key, params[key])
-        return  hmac.new(secret, sig, sha256).hexdigest()
+        return hmac.new(secret, sig, sha256).hexdigest()
 
     def url_for_get(self, path, parameters):
         return self._full_url_with_params(path, parameters)
@@ -162,7 +162,7 @@ def _auth_query(self, include_secret=False):
             return base
 
     def _signed_request(self, path, params, include_signed_request, include_secret):
-        if include_signed_request:
+        if include_signed_request and self.api.client_secret is not None:
             if self.api.access_token:
                 params['access_token'] = self.api.access_token
             elif self.api.client_id: