Remove the code that cleared "read fields set" for merged scans. That code
was based on assumption that "We're going to just read rowids", while 
actually QUICK_RANGE_SELECT code would also need key part values to check 
that retrieved record(s) fall within the scanned intervals.


mysql-test/r/index_merge_innodb.result:
  BUG#21277: Testcase
mysql-test/t/index_merge_innodb.test:
  BUG#21277: Testcase