Allow developers to define TestRequest URL params and body separately

Webtest seems to confuse URL parameters and body content when creating JSON requests.

For example:

# Test GET /api/photos?id=10
response = self.testapp.get('/api/photos/', {'id': 10})

# If I want to test PUT /api/photos?id=10 I have to do this:
response = self.testapp.put_json('/api/photos?id=10', {'title': 'Different title'})
# Because the following will PUT to /api/photos and will serialize id=10 in the JSON body.
response = self.testapp.put_json('/api/photos', {'id': 10, 'title': 'Different title'})

In my request handlers, request.get and request.json.get are not equivalent. Example:

def put(self):
  # Get the id from the URL params.
  id_param = self.request.get('id')
  # Get the id from the JSON body.
  id_val = self.request.json.get('id')

It'd be fantastic if webtest supported both as separate options.

response = self.testapp.put_json(url, params, body, headers, *args, **kwargs)

Start by tracing the TestApp put_json request helper and compare it with the GET and PUT examples in the issue. Determine how URL parameters and JSON body content are currently passed, then add separate support for both and verify that the URL query and request body remain distinct.