Add application/json as acceptable response type for documents

Not all servers are configured to return JSON-LD as application/ld+json, so requests
which only accept application/ld+json will fail with a 406 error response. Adding
application/json as an acceptable response will increase the likelyhood a request
will succeed.
Add */*; q=0.1 to Accept header and remove code duplication

Unfortunately, it is not possible to signal support for media types using the +json suffix directly. This is a work around that goes in the similar direction as #58.