PATCH support

[andy-a-o]

Is your feature request related to a problem? Please describe.
http://www.hl7.org/fhir/STU3/http.html#patch

Describe the solution you'd like
PATCH method is supported as described in the spec.

Failed tests
ConnectathonPatchTrackTest

[jjrdk]

I created a draft PR to propose a way to add support for patch.

[jjrdk]

@kennethmyhra Can we close this issue now that #341 has been merged?

[kennethmyhra]

Absolutely

[andy-a-o]

http://www.hl7.org/fhir/R4/http.html#patch

The body of a PATCH operation SHALL be either:
a JSON Patch document with a content type of application/json-patch+json
an XML Patch document with a content type of application/xml-patch+xml
a FHIRPath Patch parameters resource with FHIR Content Type

for now, it seems that only FHIR Patch was implemented. Can't really say this issue is solved... Can't re-enable tests because of that (tests send JSON patch).

[andy-a-o]

#384