Allow the page scripts to overwrite the faked functions.

Fixes #121

lib/intercept.js

@@ -99,6 +99,13 @@
 										else {
 											return original;
 										}
+									}, window),
+									set: exportFunction(function(value){
+										Object.defineProperty(
+											constructor.prototype,
+											name,
+											{value}
+										);
 									}, window)
 								}
 							);