Bring back Aeson number formatting as the default.

This default is actually quite sensible: not only does it reasonably switch
between fixed-decimal and scientific notation, it also discerns fractional
from integral numbers.

The new `Generic` option would simply print _all_ numbers in floating-point
format, which is not really acceptable, seeing as JSON is a dynamic format
and uses `Number` for numbers of any type.
E.g. 17.0 is not really an integer, is it?

Now `Generic` again prints integers as integers.

Data/Aeson/Encode/Pretty.hs

@@ -87,7 +87,9 @@ data PState = PState { pLevel     :: Int
 data Indent = Spaces Int | Tab
 
 data NumberFormat
-  -- | Use decimal notation for values between 0.1 and 9,999,999, and scientific
+  -- | The standard behaviour of the 'Aeson.encode' function. Uses
+  --   integer literals for integers (1, 2, 3...), simple decimals
+  --   for fractional values between 0.1 and 9,999,999, and scientific
   --   notation otherwise.
   = Generic
   -- | Scientific notation (e.g. 2.3e123).
@@ -195,7 +197,7 @@ fromIndent PState{..} = mconcat (replicate pLevel pIndent)
 
 fromNumber :: PState -> S.Scientific -> Builder
 fromNumber st x = case pNumFormat st of
-  Generic    -> formatScientificBuilder S.Generic Nothing x
+  Generic    -> Aeson.encodeToTextBuilder $ Number x
   Scientific -> formatScientificBuilder S.Exponent Nothing x
   Decimal    -> formatScientificBuilder S.Fixed Nothing x
   Custom f   -> f x