merged branch ShadowPrince/master (PR #1042)

This PR was squashed before being merged into the master branch (closes #1042).

Discussion
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Registration a Twig_SimpleFunction after extensions initialized throw not LogicException, but Fatal Error

Registration a Twig_SimpleFunction after extensions initialized throw not LogicException, but Fatal Error:

$environment->addFunction(new Twig_SimpleFunction(...))
after 
$environment->render(...)
throws Fatal Error: 
object of Twig_SimpleFunction could not be converted to string

Commits
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92e97b9 Registration a Twig_SimpleFunction after extensions initialized throw not LogicException, but Fatal Error