Fix/all of string prop

[LuisTejedaS]

Current behavior:
When in a defined spec you happen to have a schema defined like this

            "properties": {
              "id": {
                "allOf": [
                  {
                    "type": "string",
                    "example": "3",
                    "nullable": false,
                    "description": "A numeric identifier"
                  },
                  {
                    "nullable": false
                  }
                ]
              },

the conversion process generates a message like this

"{\n  \"id\": {\n    \"value\": \"<Error: Could not resolve allOf schema\"\n  },\n  \"name\": \"Contract.pdf\"\n}"

Expected behavior:
It should generate a message like this:

'{\n  "id": "3",\n  "name": "Contract.pdf"\n}'

Issue causes:
In resolveRefs method, we are trying to identify the type of the schema whether it is Array, object, a reference, etc.
But the default behavior is considering the schema as an object

      else {
        return {
          type: SCHEMA_TYPES.object
        };
      }

When we have a schema like the one mentioned before, the generated schema for

{
  "nullable": false
}

was

{
  type: object
}

So the allOf merger was trying to merge a schema type object into a schema type string throwing an error and cached by the process adding the message in the result: Error: Could not resolve allOf schema.

The way we resolve:
If we know that we are processing a schema that comes from an allOf parent then we are returning the same schema instead of defaulting to a schema with type as object, and no other properties. So the result of the method is the same

{
  nullable: false
}

And the merger can merge the string with this sub-schema.