Retrieve a Collection API endpoint uses request content-type instead of accept header

[carbontwelve]

Describe the bug

When using the Retrieve a Collection API endpoint as documented here I notice that when sending a request with the Accept header correctly set to application/json the server was incorrectly returning Content-Type:text/html.

Upon looking at the source code I can see this bug is seemingly due to the following line: https://github.com/writeas/writefreely/blob/master/collections.go#L321

Steps to reproduce (if necessary)

Steps to reproduce the behavior:

1. Request the GET /api/collections/{COLLECTION_ALIAS} API endpoint with Accept header set for application/json
2. Server responds with HTML.

Expected behavior

The request is stating it accepts json the server should respond with content type json or HTTP 415 Unsupported Media Type if not supported. Given that the API supports returning JSON it should have responded with JSON.

The following line https://github.com/writeas/writefreely/blob/master/collections.go#L321 potentially needs amending to include the Accept header as well.

[thebaer]

Thanks for the report!

For a first pass, we can refactor IsJSON() to accept a http.Request instead of a string and check both the Content-Type and Accept headers.

@robjloranger want to take a look?

👍