Fix invalid String#indexOf test in a coercible Date instance

Fixes #793

As described by @ediosyncratic:

Date(0) flows through the given algorithm to ToDateString, which should:

> Return an implementation-dependent String value that represents tv as
> a date and time in the current time zone using a convenient, human-readable form.

The string is implementation-dependent and there is nothing constraining it to use
a two-digit day-of-month (so "Thu Jan 1, 1970, 0:0:0" would be valid), much less to
position it at index 8.

test/built-ins/String/prototype/indexOf/S15.5.4.7_A1_T11.js

-// Copyright 2009 the Sputnik authors.  All rights reserved.
-// This code is governed by the BSD license found in the LICENSE file.
-
-/*---
-info: String.prototype.indexOf(searchString, position)
-es5id: 15.5.4.7_A1_T11
-description: Instance is Date(0) object
----*/
-
-var __instance = new Date(0);
-
-__instance.indexOf = String.prototype.indexOf;
-
-//////////////////////////////////////////////////////////////////////////////
-//CHECK#1
-if ((__instance.getTimezoneOffset()>0 ? __instance.indexOf('31') : __instance.indexOf('01')) !== 8) {
-  $ERROR('#1: __instance = new Date(0); __instance.indexOf = String.prototype.indexOf;  (__instance.getTimezoneOffset()>0 ? __instance.indexOf(\'31\') : __instance.indexOf(\'01\')) === 8. Actual: '+(__instance.getTimezoneOffset()>0 ? __instance.indexOf('31') : __instance.indexOf('01')) ); 
-}
-//
-//////////////////////////////////////////////////////////////////////////////