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/fixtures/schema.sql

@@ -443,7 +443,7 @@ CREATE TABLE complex_items (
 
 CREATE TABLE compound_pk (
     k1 integer NOT NULL,
-    k2 integer NOT NULL,
+    k2 text NOT NULL,
     extra integer
 );