SignedQuery should respect explicity body and content-type choices

A PostQuery used to override any explicityly given Body and
Content-Type choices in the SignedQuery. I belive this is to be
erroneous, as it simply overrode explicit selections due to a
PostQuery, which is particularly impossible to deal with when
implementing AWS version 4 signing.

It now enforces the PostQuery defaults *only* if the explicit
selections are missing.

Aws/Core.hs

@@ -373,21 +373,31 @@ queryToHttpRequest SignedQuery{..}
                                         , fmap (\auth -> ("Authorization", auth)) sqAuthorization]
                               ++ sqAmzHeaders
                               ++ sqOtherHeaders
-      , HTTP.requestBody = case sqMethod of
-                             PostQuery -> HTTP.RequestBodyLBS . Blaze.toLazyByteString $ HTTP.renderQueryBuilder False sqQuery
-                             _         -> case sqBody of
-                                            Nothing -> HTTP.RequestBodyBuilder 0 mempty
-                                            Just x  -> x
+      , HTTP.requestBody = 
+
+        -- | An explicityly defined body parameter should overwrite everything else.
+        case sqBody of
+          Just x -> x
+          Nothing -> 
+            -- | a POST query should convert its query string into the body
+            case sqMethod of
+              PostQuery -> HTTP.RequestBodyLBS . Blaze.toLazyByteString $ 
+                           HTTP.renderQueryBuilder False sqQuery
+              _         -> HTTP.RequestBodyBuilder 0 mempty
+
       , HTTP.decompress = HTTP.alwaysDecompress
 #if MIN_VERSION_http_conduit(1, 9, 0)
       , HTTP.checkStatus = \_ _ _ -> Nothing
 #else
       , HTTP.checkStatus = \_ _ -> Nothing
 #endif
       }
-    where contentType = case sqMethod of
-                           PostQuery -> Just "application/x-www-form-urlencoded; charset=utf-8"
-                           _ -> sqContentType
+    where 
+      -- | An explicitly defined content-type should override everything else.
+      contentType = sqContentType `mplus` defContentType
+      defContentType = case sqMethod of
+                         PostQuery -> Just "application/x-www-form-urlencoded; charset=utf-8"
+                         _ -> Nothing
 
 -- | Create a URI fro a 'SignedQuery' object. 
 --