fix(filterFilter): don't throw if key.charAt is not a function

Previously, when an object has keys which are not of type string, `filterFilter`
would throw an exception for trying to call `key.charAt()`, which is a string
method.

This commit checks whether `charAt` is defined before calling it.

Fixes #15644

Closes #15660

src/ng/filter/filter.js

@@ -226,7 +226,10 @@ function deepCompare(actual, expected, comparator, anyPropertyKey, matchAgainstA
       var key;
       if (matchAgainstAnyProp) {
         for (key in actual) {
-          if ((key.charAt(0) !== '$') && deepCompare(actual[key], expected, comparator, anyPropertyKey, true)) {
+          // Under certain, rare, circumstances, key may not be a string and `charAt` will be undefined
+          // See: https://github.com/angular/angular.js/issues/15644
+          if (key.charAt && (key.charAt(0) !== '$') &&
+              deepCompare(actual[key], expected, comparator, anyPropertyKey, true)) {
             return true;
           }
         }