Fixed: some strings and arrays in the context can break the template rendering due to PHP callables

[CamaroSS]

PHP can recognize an array or a string as a callable, causing unwanted changes to the template rendering process.
Example:

$template = new Template('Hello, {{ username }}!');
$context = new Context(['username' => 'Max']);
echo $template->render($context);

Expected:
Hello, Max!

Got:
Hello, !

It happens because 'max' is a function name in PHP, and is_callable('max') evaluates to true.
We have to be more strict here.

[idlesign]

Thank you.
Yeah, that looks like a security hole also.
We need to extend tests with that case to verify all goes fine.