Add response.headers GUC for GET/POST/PATCH/PUT/DELETE

[steve-chavez]

Custom headers improvements.

• Fixes Add headers to all responses through "response.headers" GUC #1088.
• Fixes How to access the authorization header value? #1168.
• Also added the request.method and request.path GUCs that were requested on gitter chat https://gitter.im/begriffs/postgrest?at=5dcb6b6e5eb2e813db145584.
• Aiming to Fix Override unsuitable Location header values #1427. Also thinking to allow overriding Content-Type(requested in Workaround for GET /rpc/function to STABLE function while setting response headers #1422) and Content-Range(requested in chat https://gitter.im/begriffs/postgrest?at=5dcacd06e75b2d5a19bcfb2c).

[steve-chavez]

Added the ability to override all postgrest provided headers.

[steve-chavez]

I remember we got a question in gitter chat regarding setting the http status through a response.status GUC, can't find it though. We currently support setting status through raise http://postgrest.org/en/v6.0/api.html#errors-and-http-status-codes, though that's more meant for errors and will cancel the body pgrst generates.

Should we allow overriding the http status for successful responses as well? This would make our feature set for customizable http logic complete.

Setting the response.status GUC also seems more intuitive than setting the sqlstate to our special PT*** code.

[steve-chavez]

I'll leave response.status for another time. Raising the PT*** error should be enough for most use cases.

I think this is good to merge now.