Fix for attribute methods

See 0ea157839f

The call to `merge` isn't necessary since the hashes are merged in the
call to `Buffer#attributes` with `merge_attrs`

This commit can result in different behaviour in certain case, since
the hashes are being merged with `merge_attrs` which handles `class` and
`id` attributes. Previously the value from the 'old style' would replace
the value from the 'new style`

e.g.

    %p.the_class{:class => old_attrs}(class = new_attrs)

(where `old_attrs` and `new_attrs` are methods that return 'old' or
'new')

Previously this would result in:

    <p class='old the_class'></p>

with this commit the result is:

    <p class='new old the_class'></p>

lib/haml/compiler.rb

@@ -195,7 +195,7 @@ END
         elsif attributes_hashes.size == 1
           attributes_hashes = ", #{attributes_hashes.first}"
         else
-          attributes_hashes = ", (#{attributes_hashes.join(").merge(")})"
+          attributes_hashes = ", #{attributes_hashes.join(", ")}"
         end
 
         push_merged_text "<#{t[:name]}", 0, !t[:nuke_outer_whitespace]