Overriding default JSON GET/POST behavior

Creating a ticket as per https://groups.google.com/d/msg/liftweb/E-XlChwJd10/-8Uix3KJDXEJ.

To summarize, the idea was to allow the customization of how JSON requests are handled; currently, the framework is hardcoded to require the Content-Type header of any JSON requests to always be `application/json`. However, this is not how many JSON based web services work, and shouldn't be a hardcoded requirement in the case of e.g. web services that are designed to only use JSON anyway (i.e. the content-type selection is redundant).

However YMMV, and perhaps for the sake of keeping the framework simple—although at least the HTTP REST code is not overly simple and easy to follow anyway—it might be worth to keep things how they are.