Null data on response should be wrapped in JSON object

In the `send a response` algorithm, when the data to be returned as the
response for a command is `null`, it should be wrapped in a JSON object
with a `value` property set to `null` rather than sending no body. Fixes
issue #807.

Author: jimevans

webdriver-spec.html

@@ -966,9 +966,9 @@ <h3>Processing Model</h3>
    <dd>"<code>no-cache</code>"
   </dl>
 
- <li><p>If <var>data</var> is not <a><code>null</code></a>, let <var>response</var>’s
-  <a>body</a> be a JSON <a>Object</a> with a key <code>value</code>
-  set to the <a>JSON serialization</a> of <var>data</var>.
+ <li><p>Let <var>response</var>’s <a>body</a> be a JSON <a>Object</a> with
+  a key <code>value</code> set to the <a>JSON serialization</a> of
+  <var>data</var>.
 
  <li><p>Let <var>response bytes</var> be the byte sequence resulting
   from serializing <var>response</var> according to the rules in [[!RFC7230]].