fixed and tested object has_extension
Merge branch 'object_has_extension_fix' of git://github.com/dnadesign/silverstripe-framework into 3.1

* 'object_has_extension_fix' of git://github.com/dnadesign/silverstripe-framework:
  fixed and tested object has_extension

Conflicts:
	core/Object.php
	tests/core/ObjectTest.php