Return mongoosejs error as JSON, check if PUT/POST'ed properties exist in the model

Thanks for this very nice project.
I just found it today, so please consider my change a proposal, I might have missed some places regarding validation.

I propose the following that allows clients to handle validation errors appropriately.
Maybe this could be of some help to others as well.

Regarding posting/putting properties that do not exist in the model, the following error happened:
```
TypeError: Cannot read property 'instance' of undefined
    at modifyObject (/Users/michael/code/node_modules/express-restify-mongoose/lib/express-restify-mongoose.js:258:22)
    at callbacks (/Users/michael/code/node_modules/express/lib/router/index.js:164:37)
    at ensureContentType (/Users/michael/code/node_modules/express-restify-mongoose/lib/express-restify-mongoose.js:189:13)
    at callbacks (/Users/michael/code/node_modules/express/lib/router/index.js:164:37)
    at cleanQuery (/Users/michael/code/node_modules/express-restify-mongoose/lib/express-restify-mongoose.js:102:9)
    at callbacks (/Users/michael/code/node_modules/express/lib/router/index.js:164:37)
    at param (/Users/michael/code/node_modules/express/lib/router/index.js:138:11)
    at param (/Users/michael/code/node_modules/express/lib/router/index.js:135:11)
    at pass (/Users/michael/code/node_modules/express/lib/router/index.js:145:5)
    at Router._dispatch (/Users/michael/code/node_modules/express/lib/router/index.js:173:5)
```