Fix parsing of fractional NumericDate's

[ulidtko]

Basically, the bug was this:

λ ➔ eitherDecode "123.999999999" :: Either String NumericDate 
Right (NumericDate 123999999999)

caused by a misuse of Data.Scientific.coefficient, where the corresponding base10Exponent was being blissfully disregarded (incorrectly assumed always zero, I guess):

haskell-jwt/src/Web/JWT.hs

Lines 622 to 623 in 7543f3d

	instance FromJSON NumericDate where
	parseJSON (Number x) = return $ NumericDate $ coefficient x

I.e. aeson parses the number as expected — into 123999999999e-9:

λ ➔ (123.999999999 :: Scientific) == scientific 123999999999 (-9)
True

but the instance just drops the exponent on the floor. 🤷

Resolves #4.

[ulidtko]

Hey @puffnfresh, thanks! Would be also great to merge #9, please take a look 🙏