Fix Optional ReqBody' (wrap value into Maybe)

[unclechu]

Closes #1346

Here is a small reproducible demonstration of the fix (requires Nix to run):
https://gist.github.com/unclechu/77ec314b8651c0c17ea4ac486fdb9804

ReqBody' '[Optional, Strict] '[JSON] [Integer]

Is resolving to [Integer] in server serializer. This fix makes it become Maybe [Integer] instead (because it has Optional mode).

This PR brings these changes:

1. Makes Optional ReqBody' wrap it’s type into Maybe
2. Request without headers and without payload resolves to Nothing
3. Request with proper Content-Type but without payload resolves to Nothing
4. When proper Content-Type header is provided and payload is there it resolves Just in case there were no parsing errors
5. Required ReqBody' behaves as usual (not affected by this fix)

[unclechu]

I’m not certain about these two patterns:

(SFalse, STrue,  False) -> return . either (const Nothing) (Just . Right)
(SFalse, SFalse, False) -> return . either (const Nothing) Just

But they seem to work as expected. It turned ouit that requests from the tests are having requestBodyLength request as KnownLength 0. I’m not sure why.

[dalpd]

Hi!

Is there something keeping this from going upstream? I was under the impression this was the default behavior (that one would construct a custom type with '[Optional] to have optional request bodies like how Header works), but as pointed out that's not the case.

[tchoutri]

Yep', there is a conflict to be resolved in this PR. :)

[unclechu]

@tchoutri I resolved the conflicts and rebased the change on top of latest master. Also I ran the tests locally with a successful result. Please have a look.

[dalpd]

Is there anything we can do to help maintainers move this PR forward?

[tchoutri]

I somehow managed to miss the mention from 12 days ago. There could be a CI failure due to HsOpenSSL that I'm exploring in #1635, otherwise I don't have anything more to ask. :)

[tchoutri]

@unclechu okay, I believe that you should bump the version of HsOpenSSL used, reading its changelog: https://flora.pm/packages/%40hackage/HsOpenSSL/0.11.7.5/changelog

[isomorpheme]

I'd like to see this PR merged - is there something I can do to help? I guess if I wanted to rebase it myself I'd have to open a separate PR?