Allow overriding of JSON deserialization length for unit testing.

- Made JavaScriptSerializer constructor public, allowing overriding of
  parameters.
- Allow using a different instance of JavaScriptSerializer in
  BrowserResponseBodyWrapperExtensions.DeserializeJson<TModel>().
Merge pull request #1149 from rlipscombe/testing-maxjsonlength

Allow overriding of JSON deserialization length for unit testing.