Fixed semicolon being appended to filename for BinaryResult(s) using Content-Disposition "attachment"

While working on a website I noticed that my files had a semicolon being added to the end of them. The headers showed the following: `Content-Disposition: attachment; filename=XXXX.mp3;`

There shouldn't be the trailing semicolon, so this pull request simply removes it. This doesn't appear to break anything when running the main tests:

    $ go test github.com/robfig/revel
    ok  	github.com/robfig/revel	0.117s

If more tests are needed, I'd be happy to attach them.