Check that spy.proxy is an actual spy before use

This prevents problems when the object spied upon has a `proxy` method that isn't a spy.  For example, controllers from Spine.js.

lib/sinon-chai.js

@@ -39,7 +39,9 @@
     function getMessages(spy, action, nonNegatedSuffix, always, args) {
         var verbPhrase = always ? "always have " : "have ";
         nonNegatedSuffix = nonNegatedSuffix || "";
-        spy = spy.proxy || spy;
+        if (isSpy(spy.proxy)) {
+            spy = spy.proxy;
+        }
 
         function printfArray(array) {
             return spy.printf.apply(spy, array);