Fix Generic encoding of records with single Maybe field

[Lysxia]

With

data Single = Single { f :: Maybe Int } deriving Generic

instance ToJSON Single where
  toJSON = genericToJSON defaultOptions{omitNothingFields=True,unwrapUnaryRecords=True}

We have:

encode (Nothing :: Maybe Int) = "null"
encode (Just 1 :: Maybe Int) = "1"

Yet:

encode (Single Nothing) = "{}"
encode (Single (Just 1)) = "1"

That is, if the field is Nothing, then the record is not being unwrapped and omitNothingFields applies instead, producing an empty object.

[bergmark]

Looks great, thanks!

To be safe I think I'll include this in 1.2.0.0. Do you have a preferred order for merging your PR's?

[Lysxia]

Actually I fixed this in the refactoring at the same time (which is how I came across the issue of this PR), so I would merge the refactoring (#524) before this one which would then only provide the test.

#524, #532, and finally #522 (after one last change to do I mentioned over there).

[bergmark]

This has some merge conflicts now

[Lysxia]

Rebased. This now just adds the regression test case.

[bergmark]

Great!