Allow variables within strings in BooleanParser

[simonschaufi]

The following test case currently is not possible as the boolean parser either accepts strings OR variables but not both.

['\'{foo}\' == \'{bar}\'', true, ['foo' => 'a', 'bar' => 'a']],

I would expect this to be true. A workaround is to assign the string to a variable before with the f:variable-ViewHelper and then use the new created variable for comparison.