RFC6902 enabled

[fairsayan]

I implemented RFC6902 as suggested in the #224 issue.
Some caveats:

1. mongoose-json-patch must be pluged by the user in schema definition. I tried to plug it addressing model.schema inside express-restify-mongoose module without success.
2. The structure of this module is a chain of middlewares and it was too hard for me to distinguish the behavior between PATCH with application/json and the one with application/json-patch+json. So in my implementation PATCH with application/json doesn't use findOneAndUpdate even if the correspondent option is true because I needed to run the filterAndFindById instead.
3. To use this implementation bodyParser.json has to accept application/json-patch+json content type as well so in the application must be declared something like:
   router.use (bodyParser.json({strict:false, type: (req) => {
   var ct = req.headers['content-type'];
   return (ct === 'application/json-patch+json') || (ct === 'application/json');
   }}));
   otherwise req.body will be empty and won't be applied any patch.

[Zertz]

What happens if the user doesn't have mongoose-json-patch installed?

[fairsayan]

@Zertz a big crash