util: ensure that the .inspect function isn't the one being executed

Fixes #2225.

lib/util.js

@@ -155,7 +155,7 @@ function formatValue(ctx, value, recurseTimes) {
   // Check that value is an object with an inspect function on it
   if (value && typeof value.inspect === 'function' &&
       // Filter out the util module, it's inspect function is special
-      value !== exports &&
+      value.inspect !== exports.inspect &&
       // Also filter out any prototype objects using the circular check.
       !(value.constructor && value.constructor.prototype === value)) {
     return value.inspect(recurseTimes);

test/simple/test-util-inspect.js

@@ -75,3 +75,7 @@ assert.doesNotThrow(function () {
   r.toString = null;
   util.inspect(r);
 });
+
+// GH-2225
+var x = { inspect: util.inspect };
+assert.ok(util.inspect(x).indexOf('inspect') != -1);