[DONT MERGE] Always take reference when accessing value passed as argument to a macro

[msrd0]

This was proposed by @djc to fix #708 - but just blindly taking references does not seem to work

[djc]

Sorry, I don't have much time for this so if you want more help can you comment here with some of the generated code and point out where/why the problem happens?

[msrd0]

It fails because it cannot compare &str with str (i.e. when the macro argument was already a borrowed string, it becomes a double borrow which cannot be compared to a literal anymore). You can see it in the CI here: https://github.com/djc/askama/runs/7705017639?check_suite_focus=true