Support the prefer headers

[brianpos]

http://hl7.org/implement/standards/fhir/search.html#errors

• Prefer: handling=strict: Client requests that the server return an error for any unknown or unsupported parameter
• Prefer: handling=lenient: Client requests that the server ignore any unknown or unsupported parameter

http://hl7.org/implement/standards/fhir/http.html#prefer

• Prefer: return=minimal
• Prefer: return=representation
• Prefer: return=OperationOutcome

http://hl7.org/implement/standards/fhir/async.html#3.1.6.2.0.1

• Prefer: respond-async

[brianpos]

Just a note that the minimal/representation/op outcome are already handled by the controller, so its really just the other 2, handling and respond-Async that are in question here.
(have posted the question to the fhir chat too)

[brianpos]

Per https://tools.ietf.org/html/rfc7240 section 2, either comma-separated or two separate Prefer headers are both accepted. (Thanks Paul Church)