Add endpoint to create gist

[matsubara0507]

ref https://docs.github.com/rest/reference/gists#create-a-gist

ghci> github (OAuth "...") $ createGistR (NewGist "" (HashMap.fromList [("hoge.txt", NewGistFile "abc")]) False)
Right (Gist {...})

about NewGist's fields:

• description is optional param according to reference. But, when use Maybe type and Nothing, occur error Invalid request.\\n\\nFor 'properties/description', nil is not a string.. So, description param is Text type.
• public is optional param according to reference. But, when use Maybe type and Nothing, occur error Left (ParseError "Error in $.public: expected Bool, but encountered Null"). So, public param is Bool type.

[phadej]

{ "foo": null }

is not the same as

{}
```.

See https://github.com/phadej/github/blob/master/src/GitHub/Data/Milestone.hs#L56-L65 for an example (filtering out `null`s).

[phadej]

brackets are redundant

[matsubara0507]

thx, remove () 91424a9

[matsubara0507]

Thank you. Use Maybe a type to description and public params with filter notNull. c44824b