postgest: apply xhr config before additional xhr hook

src/postgrest.js

@@ -110,11 +110,20 @@ export default (api_root, auth={}) => {
 
 			// 2. return values on Prefer
 			//			if (['POST', 'PATCH'].includes(options.method) && options.headers && !options.headers.Prefer)
-			if (['POST', 'PATCH'].includes(options.method))
+			if (['POST', 'PATCH'].includes(options.method)) {
+				// if the config is already set
+				let _old_config = options.config
+
 				options.config = xhr => {
+					// apply the provided xhr config
+					let _old_config_xhr = _old_config(xhr)
+					if (_old_config_xhr)
+						xhr = _old_config_xhr
+
 					xhr.setRequestHeader('Prefer', 'return=representation')
 					return xhr
 				}
+			}
 
 			// normalize params: since options.params is an object, it cannot support
 			// duplicated query keys, while postgrest supports that.