BugFix: Fix underlines in HTML copy.

The CSS declaration for underline (and overline and strike-through) is
introduce by the words "text-decoration" NOT "font-decoration", this commit
fixes that error. https://bugs.launchpad.net/mudlet/+bug/1130450 raised
this issue and is solved by it.

Signed-off-by: Stephen Lyons <slysven@virginmedia.com>