[Feature #498] test/cgi/test_cgi_tag_helper.rbにテストを追加

Feature #498: test/cgi/test_cgi_tag_helper.rbe$B$K%F%9%H$rDI2Ce(B
http://redmine.ruby-lang.org/issues/show/498

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e$B%A%1%C%He(B #498 e$B$,99?7$5$l$^$7$?!#e(B (by Yui NARUSE)

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Applied in changeset r18866.

http://redmine.ruby-lang.org/issues/show/498