Correctly infer types on document arrays

[JavaScriptBach]

Summary

Fix #12882.

The key insight here is that arrays of scalars and arrays of objects have different type inference logic. For example:

1. type: [{type: String, required: true}] is an array of strings.
2. type: [{type: "String", required: true}] is also an array of strings.
3. type: [{type: {foo: {type: String}, required: true}] is an array of objects that contains a field named type. Note here that type must be interpreted as a real key, not a type key.

So if we have an array, we need to check if it's an array of scalars or objects. This is done as follows:

1. If the object has a type key that isn't callable and isn't a string, it's an object, so we need to call ObtainDocumentType to make sure that its keys are interpreted correctly. (i.e. if the object has a key called type, this is a real key, not a type key)
2. In all other cases, we continue to assume that the array is a scalar.

Examples

Adds tests.

[vkarpov15]

LGTM, just one minor question/suggestion.

[vkarpov15]

This looks like it also needs to handle the case where Item[TypeKey] is a string. type: 'String' is equivalent to type: String.

[JavaScriptBach]

Good point. I handled this case and added tests