title: got 'invalid_grant' in oauth2 SignedJwtAssertionCredentials
body: <p>I am trying to make an oauth2 access_token in a server-to-server JSON API scenario. But it failed with invalid_grant error, please help.</p>
  
  <pre><code>from oauth2client.client import SignedJwtAssertionCredentials
  
  KEY_FILE = 'xxxxxxxxxxxx-privatekey.p12'
  
  with open(KEY_FILE, 'r') as fd:
      key = fd.read()
  
  SERVICE_ACCOUNT_EMAIL = 'xxxxxx.apps.googleusercontent.com'
  
  credentials = SignedJwtAssertionCredentials(SERVICE_ACCOUNT_EMAIL, key,
        scope="https://www.googleapis.com/auth/datastore https://www.googleapis.com/auth/userinfo.email",
        token_uri='https://accounts.google.com/o/oauth2/token')
  
  
  assertion = credentials._generate_assertion()
  
  h = httplib2.Http()
  credentials._do_refresh_request(h.request)
  </code></pre>
  
  <p>and I got</p>
  
  <pre><code>Traceback (most recent call last):
    File "/Users/pahud/Projects/oauth2client/x.py", line 24, in &lt;module&gt;
      credentials._do_refresh_request(h.request)
    File "/Users/pahud/Projects/oauth2client/oauth2client/client.py", line 710, in _do_refresh_request
      raise AccessTokenRefreshError(error_msg)
  oauth2client.client.AccessTokenRefreshError: invalid_grant
  [Finished in 0.7s with exit code 1]
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/iGGYx.png" rel="nofollow noreferrer">http://i.stack.imgur.com/iGGYx.png</a></p>