accesskeys: combine using correct case

We always thought, or claimed that we combined accesskeys based on
prefering the case before using an alternate case.

It seems that our code actually prefered uppercase and then would try
lowercase.  Thus this string:

"Close Other Tabs" combined with an accesskey of "o" would result in
"Close &Other Tabs" because capital O would win even though lowercase O
was present.  The correct combineation should have been "Cl&ose Other
Tabs", preferring the correct case before the alternate case.