URL-encode Location header (closes #588)

The database returns an array of key-value strings like `"k1=eq.hello
world"`. Haskell URL-encodes the portion after the equal sign and joins
them with `&`.

Includes updates to tests in Feature.InsertSpec: The CompoundPK has been
modified to have one Int and one String. We attempt to add a key with a
String that has spaces and other special characters. This requires that
the returned Location header is properly URL-encoded.

test/TestTypes.hs

@@ -37,9 +37,9 @@ instance JSON.FromJSON IncPK where
 
 data CompoundPK = CompoundPK {
   compoundK1 :: Int
-, compoundK2 :: Int
+, compoundK2 :: String
 , compoundExtra :: Maybe Int
-}
+} deriving (Eq, Show)
 
 instance JSON.FromJSON CompoundPK where
   parseJSON (JSON.Object r) = CompoundPK <$>