merged branch arnaud-lb/ext (PR #530)

Commits
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569f782 fix ZTS build
1606910 Fix undefined behavior
0ce8d3c Convert item argument to string
d807981 don't try to access non public property or method
7e566b8 test twig_template_get_attributes

Discussion
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twig_template_get_attributes tests and fixes

This enables testing of twig_template_get_attributes (every TemplateTest test is ran against twig_template_get_attributes too) and fixes some bugs I've found.

There is still 3 failing tests due to a difference of how twig_template_get_attributes and Twig_Template::getAttribute use Twig_Template::$cache.

Twig_Template::getAttribute doesn't cache properties, and twig_template_get_attributes expects $cache[$class]['properties'] to be set if $cache[$class] is set. So twig_template_get_attributes can fail if the cache is already populated by Twig_Template::getAttribute.