util,test: Use consistent Date representation for util/inspect

Re: #4314
`Inspect` formats dates in two different ways, depending on
whether it has properties or not.

No properties and it uses `toString`.
https://github.com/nodejs/node/blob/master/lib/util.js#L272
> The toString() method always returns a string representation of
> the date in American English.

While when properties are present it uses `toUTCString`.
https://github.com/nodejs/node/blob/master/lib/util.js#L393

> The format of the return value may vary according to the platform.
> The most common return value is a RFC-1123 formatted date stamp,
> which is a slightly updated version of RFC-822 date stamps.

I am wondering why two different representations are used?

And being 2015, it would seem sensible to use `toISOString`.

> The toISOString() method returns a string in simplified
> extended ISO format (ISO 8601), which is always 24 characters
> long: YYYY-MM-DDTHH:mm:ss.sssZ. The timezone is always zero
> UTC offset, as denoted by the suffix "Z".

./configure
make test
Total errors found: 0

lib/util.js

@@ -269,7 +269,7 @@ function formatValue(ctx, value, recurseTimes) {
       return ctx.stylize(RegExp.prototype.toString.call(value), 'regexp');
     }
     if (isDate(value)) {
-      return ctx.stylize(Date.prototype.toString.call(value), 'date');
+      return ctx.stylize(Date.prototype.toISOString.call(value), 'date');
     }
     if (isError(value)) {
       return formatError(value);
@@ -390,7 +390,7 @@ function formatValue(ctx, value, recurseTimes) {
 
   // Make dates with properties first say the date
   if (isDate(value)) {
-    base = ' ' + Date.prototype.toUTCString.call(value);
+    base = ' ' + Date.prototype.toISOString.call(value);
   }
 
   // Make error with message first say the error

test/parallel/test-util-inspect.js

@@ -13,7 +13,7 @@ assert.equal(util.inspect(undefined), 'undefined');
 assert.equal(util.inspect(null), 'null');
 assert.equal(util.inspect(/foo(bar\n)?/gi), '/foo(bar\\n)?/gi');
 assert.equal(util.inspect(new Date('Sun, 14 Feb 2010 11:48:40 GMT')),
-  new Date('2010-02-14T12:48:40+01:00').toString());
+  new Date('2010-02-14T12:48:40+01:00').toISOString());
 
 assert.equal(util.inspect('\n\u0001'), "'\\n\\u0001'");
 
@@ -220,7 +220,7 @@ assert.equal(util.inspect(value), '{ /123/gi aprop: 42 }');
 // Dates with properties
 value = new Date('Sun, 14 Feb 2010 11:48:40 GMT');
 value.aprop = 42;
-assert.equal(util.inspect(value), '{ Sun, 14 Feb 2010 11:48:40 GMT aprop: 42 }'
+assert.equal(util.inspect(value), '{ 2010-02-14T11:48:40.000Z aprop: 42 }'
 );
 
 // test the internal isDate implementation
@@ -313,6 +313,12 @@ assert.doesNotThrow(function() {
   util.inspect(d);
 });
 
+assert.doesNotThrow(function() {
+  var d = new Date();
+  d.toISOString = null;
+  util.inspect(d);
+});
+
 assert.doesNotThrow(function() {
   var r = /regexp/;
   r.toString = null;