use lodash to check types in resolver.js

[ponelat]

fixes swagger-api/swagger-ui#1349
The issue was, when 'required: null' we had a stack overflow.
The problem was in how we checked for objects.... javascript has this delightful way of classifying null as an object.

typeof null === 'object' // Thanks javascript

Which is why we often see

if(something && typeof something === 'object') { ... } // first make sure it isn't null

I've scanned the source, and replaced instances of typeof something === 'object' to _.isObject(someting).