Fixes inconsistency in @collection :root elements for XML output

This is filed in response to https://github.com/nesquena/rabl/issues/179

Basically, `@collection @entity, :root => <root_name>` or `@collection :entity => <root_name>`  is supposed to define the root name, which works for `json` but is ignored for `xml` output. Tests also assumes that root name is pluralized.