Slightly improves HAML attribute parsing

[Qqwy]

This ensures that the attribute parsing goes a little nicer, and at least does not break on the access operator . anymore.

Note that nested maps are not possible by this PR, because for this, an overhaul of the way Calliope parses attributes (Calliope.Parser.build_attributes) is necessary that uses LR or LL parsing instead of string replacements.