Fix TagNode#object_reference? for haml 4.1+

Version 4.1.0.beta.1 of the `haml` gem changed the parsing of tag nodes
without object references to set the `:object_ref` key of the value to
the symbol `:nil` instead of the string `'nil'`.

Fix this by always converting to a string before comparing.

Change-Id: I7ec6a5ab51fad4358274c1f2932ffbd1720d0328
Reviewed-on: http://gerrit.causes.com/44719
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>