mysql-test/r/update.result:
  Testcase for BUG#15935
mysql-test/t/update.test:
  Testcase for BUG#15935
sql/sql_update.cc:
  BUG#15935: 
  - Do account for the fact that used_index!=MAX_KEY is also true for cases
    when quick select is used, and use quick select then (and not full index scan).
  - Also removed the redundant "used_index= MAX_KEY" statement