Mongo's _id normalization fix

Check the type of _id before converting into string.
This is necessary for models with integer id.

The issue has been fixed in v0.9.X ( balderdashy/sails-mongo/pull/71 ) but apparently missed out during migration to v0.10.X.
Merge pull request #184 from LukaszKrawczyk/patch-1

Mongo's _id normalization fix