adds missing require, and cleans a bit #compute_public_path

compute_public_path asks is_uri?(source) a few times to avoid touching explicit URLs. This refactor moves that condition to the top to make this more obvious. Also, it cleans the method a bit now that we are on it.

I think it reads better with this patch.

Moreover, asset_tag_helper.rb uses present? and blank? in a few spots, but the corresponding require is missing.