title: create_hook Validation Error 422
body: <p>I have been playing with the parameters for awhile now but I continue to get the same 422 Validation Error. I am using runscope to test if my webhooks work. </p>
  
  <pre><code>url_path = "https://k1lavjuzlcvj.runscope.net"
  
  repo.create_hook(name="testhook",config={"url":url_path, "content_type":"json"})
  </code></pre>
  
  <p>This is the stack output that I receive from this call </p>
  
  <pre><code>09-29 13:35 github3      DEBUG    POST https://github.umn.edu/api/v3/repos/umn-csci-2041F14/testbalas/hooks with {"name": "testhook", "active": true, "config": {"content_type": "json", "url": "https://k1lavjuzlcvj.runscope.net"}, "events": ["push"]}, {}
  09-29 13:35 requests.packages.urllib3.connectionpool DEBUG    "POST /api/v3/repos/umn-csci-2041F14/testbalas/hooks HTTP/1.1" 422 113
  09-29 13:35 logger       ERROR    Failure while setting hook for testbalas:
  Traceback (most recent call last):
    File "./createurlhooksbyuid", line 107, in &lt;module&gt;
      repo.create_hook(name="testhook",config={"url":args.url, "content_type":"json"})
    File "/Users/nate/Git-Projects/administration/dependencies/github3.py/github3/decorators.py", line 38, in auth_wrapper
      return func(self, *args, **kwargs)
    File "/Users/nate/Git-Projects/administration/dependencies/github3.py/github3/repos/repo.py", line 613, in create_hook
      json = self._json(self._post(url, data=data), 201)
    File "/Users/nate/Git-Projects/administration/dependencies/github3.py/github3/models.py", line 100, in _json
      if self._boolean(response, status_code, 404) and response.content:
    File "/Users/nate/Git-Projects/administration/dependencies/github3.py/github3/models.py", line 121, in _boolean
      raise GitHubError(response)
  github3.models.GitHubError: 422 Validation Failed
  </code></pre>
  
  <p>Any help would be appreciated, thank you! </p>