Honor Accept header

Overview:
Currently edge-orders always returns the gobi xml response. We need to open that up to be much more flexible.

Approach
simply pass the provided accept header when proxying to the backend and let the backend process or reject the request

• For errors in the edge API, use the error responses you've defined here (for now)
• Create a JIRA for implementing error templates, specified via configuration
• Return all responses from the backend as-is - success, failure, etc.
• Responses from the backend should not be parsed or validated
• The RAML cannot specify schemas for any responses since they will differ depending on the 'type' query argument.
  1. If the backend returns text/plain, wrap in the XML/JSON error response format
  2. Story for mod-gobi to return error XML conforming to the schema currently in use - currently returns text/plain
  3. Story for edge-orders to remove the wrapping and return responses as-is (depends on the mod-gobi work above)
  4.Models and scheams for gobi order responses (success) can be removed - no longer needed
  5.For now, leave the examples in RAML - remove as part of #3

Acceptance criteria:

• logic updated
• unit tests updated