Fix arity warning for template handlers

[localhostdotdev]

Summary

The warning displayed a generic Class string instead of the class name (e.g. Coffee::Rails::TemplateHandler).

This makes it difficult to know which template handler is responsible for the warning and thus which one needs to be updated.

extensions could also be used and may lead to a better error message.

Other Information

handler.class # => Class

handler.to_s # => Coffee::Rails::TemplateHandler

Change:
  >> Class#call(template)
To:
  >> Class#call(template, source)

to

Change:
  >> Coffee::Rails::TemplateHandler.call(template)
To:
  >> Coffee::Rails::TemplateHandler.call(template, source)

A test would be great, not sure how I would do that though (either using an old gem version or mocking a template handler). At least it works in my manual testing.

I also fixed the instance method call to a class method call as this is how Coffee::Rails::TemplateHandler handles it:

Coffee::Rails::TemplateHandler.call(OpenStruct.new(source: 'a = 1'), 'a = 1')

[localhostdotdev]

cc: @tenderlove as you seem be the one who knows the most about this (💜💙💚💛🧡❤️)

[connorshea]

This’ll help me a lot with finding where my only remaining Rails 6 depreciation warning is, thank you! ❤️