Schema from shared `responses` object doesn't resolve for HTTP response codes

[hassansin]

Test code:

var obj = {
  "swagger" : "2.0",
  "info" : {
    "version" : "0.0.0",
    "title" : "Simple API"
  },
  "responses": {
    "Success": {
      "description": "Success",
      "schema": {
        "type": "string"
      }
    },
    "Error": {
      "description": "Error",
      "schema": {
        "type": "string"
      }
    }
  },
  "paths" : {
    "/" : {
      "get" : {
        "parameters" : [ ],
        "responses" : {
          "200" : {
            "$ref": "#/responses/Success"
          },
          "default": {
            "$ref": "#/responses/Error"
          }
        }
      }
    }
  },
  "definitions" : { }
}

var client = require('swagger-client');
var swagger = new client({
  spec:  obj,
  success: function() {
    console.log(swagger.default.operations.get.responses['200']) //undefined
    console.log(swagger.default.operations.get.responses['default']) //OK
  }
});