Add support to Haml::Plugin for Rails 6
Add support to Haml::Plugin for Rails 6

In Rails 6 the API for template handlers is changing, a template handler
must now take two arguments [1], the template and the source, otherwise you
will see the following deprecation warning:

```
ActiveSupport::DeprecationException: DEPRECATION WARNING: Single arity
template handlers are deprecated.  Template handlers must now accept two
parameters, the view object and the source for the view object.
Change:
  >> Class#call(template)
To:
  >> Class#call(template, source)
```

I have attempted to retain support for older versions of Rails by
providing a default source of `nil`.

[1] https://www.github.com/rails/rails/commit/28f88e0074

haml/haml,haml/haml,haml/haml,haml/haml