Fixed String subclassing

Atring.allocate should not ignore sublasses and should
pass `self` as `klass` to `W_Object.__init__`

Untagged specs:
 * fails:String.new returns an instance of a subclass
 * fails:String.new is called on subclasses
Removed 'fails' tag from passing spec

This was fixed in ad97fbbe652f0bdff70069fe43ed7b0f65b1fee8
Failing example used subclass of String while such subclassing
was broken.
simplifiy and fix jittests
Merge pull request #1 from topazproject/tim/string_new

simplifiy and fix jittests
Reverted JIT fixes to master version
Merge pull request #536 from krekoten/string_new

Fixed String subclassing