Fix headers parsing

[maxenced]

With raml like :

/test:
  get:
    description: test

    responses:
        200:
          headers:
            Access-Control-Allow-Origin:
              example: "*"
          body:
            application/json:
...

current version of raml-mock-service fails with :

TypeError: response.headers.forEach is not a function
    at handler (/usr/lib/node_modules/raml-mock-service/raml-mock-service.js:104:24)
    at /usr/lib/node_modules/raml-mock-service/node_modules/osprey-resources/osprey-resources.js:58:20
    at Array.forEach (native)
    at createResource (/usr/lib/node_modules/raml-mock-service/node_modules/osprey-resources/osprey-resources.js:57:13)
    at /usr/lib/node_modules/raml-mock-service/node_modules/osprey-resources/osprey-resources.js:33:7
    at Array.forEach (native)
    at createResources (/usr/lib/node_modules/raml-mock-service/node_modules/osprey-resources/osprey-resources.js:32:15)
    at ospreyResources (/usr/lib/node_modules/raml-mock-service/node_modules/osprey-resources/osprey-resources.js:17:10)
    at ospreyMockServer (/usr/lib/node_modules/raml-mock-service/raml-mock-service.js:20:10)
    at createServer (/usr/lib/node_modules/raml-mock-service/raml-mock-service.js:34:11)

This is because response.headers is a hash, not an Array.
This commit fix this by iterating correctly through the hash.