Throw an error for key actions taking multiple characters in IE

The key actions as defined by the W3C WebDriver Specification take a JSON
payload that requires the resulting key for the key down or key up action
to resolve to a single Unicode code point or a single grapheme cluster
(what appears to be a single logical "character" from a user's
perspective). The IE driver will now evaluate the item passed into the key
down or key up action, and, taking combining characters into account,
attempts to make sure there is only a single grapheme present, if more
than one Unicode code point is sent. If the entered string would results
in multiple grapheme clusters, the driver now returns an invalid argument
error, as per the spec.