[BUGFIX] Prevent spaces in BE shortcut to be replaced by + char

Due to some changes in Prototype.js the serialized form values
get wrongly escaped with the result that all spaces are replaced
by + chars.

This patch changes the used serialization of the form so the value
is correctly posted.

Resolves: #59950
Releases: 6.3, 6.2
Change-Id: I65d89ab741ae77f5bbef9421923b3878f2295570
Reviewed-on: https://review.typo3.org/31276
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters