Only use event.which when key identifier is non-ASCII

Previously event.which was used when it was less than the key identifier
which broke fn-delete since that generates a valid key identifier but a
lower event.which value that does not translate to 'delete' using the char
code converter.

Close #611

src/app/keymap.coffee

@@ -150,7 +150,7 @@ class Keymap
     if event.originalEvent.keyIdentifier.indexOf('U+') == 0
       hexCharCode = event.originalEvent.keyIdentifier[2..]
       charCode = parseInt(hexCharCode, 16)
-      charCode = Math.min(event.which, charCode) if event.which?
+      charCode = event.which if !@isAscii(charCode) and @isAscii(event.which)
       key = @keyFromCharCode(charCode)
     else
       key = event.originalEvent.keyIdentifier.toLowerCase()
@@ -171,6 +171,9 @@ class Keymap
 
     [modifiers..., key].join('-')
 
+  isAscii: (charCode) ->
+    0 <= charCode <= 127
+
   keyFromCharCode: (charCode) ->
     switch charCode
       when 8 then 'backspace'