Unify class methods definitions.

The [RDoc is not being generated for open](http://ruby-doc.org/stdlib-2.1.2/libdoc/tempfile/rdoc/Tempfile.html) (the documentation is readable only in the [yard](http://yard.ruby-doc.org/stdlib-2.1.2/Tempfile.html#open-class_method) version). This change should make it work.