Fix .:? behavior for haskell#83

It makes (.:?) return Just value if the key is present for values of type Maybe a.
Earlier {"key": null} .:? "key" would return Nothing although the key is there. It was not possible to distinguish the cases when key is absent and when value is null.
Now it treats Maybe same way as other types and returns Just Nothing.

Data/Aeson/TH.hs

@@ -100,7 +100,8 @@ import Data.Aeson.Types ( Value(..), Parser
                         , defaultTaggedObject
                         )
 import Data.Aeson.Types.Internal (Encoding(..))
-import Data.Bool           ( Bool(False, True), otherwise, (&&) , not)
+import Control.Monad       ( return, mapM, liftM2, fail, join )
+import Data.Bool           ( Bool(False, True), otherwise, (&&), not )
 import Data.Either         ( Either(Left, Right) )
 import Data.Eq             ( (==) )
 import Data.Function       ( ($), (.), flip )
@@ -953,7 +954,7 @@ instance (FromJSON a) => LookupField a where
           Just v  -> parseJSON v
 
 instance (FromJSON a) => LookupField (Maybe a) where
-    lookupField _ _ = (.:?)
+    lookupField _ _ obj key = join <$> obj .:? key
 
 unknownFieldFail :: String -> String -> String -> Parser fail
 unknownFieldFail tName rec key =

Data/Aeson/Types/Generic.hs

@@ -20,7 +20,7 @@
 module Data.Aeson.Types.Generic ( ) where
 
 import Control.Applicative ((<*>), (<$>), (<|>), pure)
-import Control.Monad ((<=<))
+import Control.Monad ((<=<), join)
 import Control.Monad.ST (ST)
 import Data.Aeson.Encode.Builder (emptyArray_)
 import Data.Aeson.Encode.Functions (builder)
@@ -656,8 +656,8 @@ instance (Selector s, GFromJSON a) => FromRecord (S1 s a) where
     {-# INLINE parseRecord #-}
 
 instance (Selector s, FromJSON a) => FromRecord (S1 s (K1 i (Maybe a))) where
-    parseRecord _ (Just lab) obj = (M1 . K1) <$> obj .:? lab
-    parseRecord opts Nothing obj = (M1 . K1) <$> obj .:? pack label
+    parseRecord _ (Just lab) obj = (M1 . K1) . join <$> obj .:? lab
+    parseRecord opts Nothing obj = (M1 . K1) . join <$> obj .:? pack label
         where
           label = fieldLabelModifier opts $
                     selName (undefined :: t s (K1 i (Maybe a)) p)

Data/Aeson/Types/Instances.hs

@@ -1525,7 +1525,7 @@ obj .: key = case H.lookup key obj of
 (.:?) :: (FromJSON a) => Object -> Text -> Parser (Maybe a)
 obj .:? key = case H.lookup key obj of
                Nothing -> pure Nothing
-               Just v  -> parseJSON v <?> Key key
+               Just v  -> Just <$> parseJSON v <?> Key key
 {-# INLINE (.:?) #-}
 
 -- | Helper for use in combination with '.:?' to provide default