refactor: plan media type

[steve-chavez]

Problem

Previously it was impossible to have doctests for the MediaType module due to recursion:

postgrest/src/PostgREST/MediaType.hs

Lines 23 to 38 in 78821a8

	data MediaType
	= MTApplicationJSON
	| MTSingularJSON
	| MTGeoJSON
	| MTTextCSV
	| MTTextPlain
	| MTTextXML
	| MTOpenAPI
	| MTUrlEncoded
	| MTOctetStream
	| MTAny
	| MTOther ByteString
	| MTPlan MTPlanAttrs
	deriving Eq
	data MTPlanAttrs = MTPlanAttrs (Maybe MediaType) MTPlanFormat [MTPlanOption]

Basically a MediaType was able to contain another MediaType. See the MTPlan constructor.

This caused the deriving instance Show ... setup to fail.

Solution

Eliminate recursion by having non-plan media types as a separate type(NormalMedia).

Some doctests are now added for MediaType.

[steve-chavez]

Here I tried adding a space like so:

Suggested change

	-- >>> decodeMediaType "application/vnd.pgrst.plan;for=\"application/json\""
	-- MTPlan (MTPlanAttrs (Just MTApplicationJSON) PlanText [])
	-- >>> decodeMediaType "application/vnd.pgrst.plan; for=\"application/json\""
	-- MTPlan (MTPlanAttrs (Just MTApplicationJSON) PlanText [])

And the decoding failed to get Just MTApplicationJSON (got Nothing instead). Although this clearly works as proven on the feature specs:

postgrest/test/spec/Feature/Query/PlanSpec.hs

Line 119 in 78821a8

r <- request methodGet "/projects" (acceptHdrs "application/vnd.pgrst.plan+json; for=\"application/json\"; options=verbose") ""

I thought there must be something wrong with whitespace on the doctests. This would have been easy to prove with the repl. But #2627 continues to be a problem.