collation

By default constant strings in second parameter of date_time() have case
insensitive collation. Because of this expressions date_format(f,'%m') and 
date_format(f,'%M') wrongly becomes equal, which results in choosing wrong 
column to sort by.

Now if second parameter of date_format() is constant then it's collation is 
changed to case sensitive.


sql/item_timefunc.cc:
  Fix bug #14016 date_format() 2nd parameter was compared using case insensitive collation.
  If second parameter of date_format() is constant then it's collation is changed to case sensitive.
mysql-test/r/date_formats.result:
  Test case for bug#14016 2nd parameter was compared using case insensitive collation
mysql-test/t/date_formats.test:
  Test case for bug#14016 2nd parameter was compared using case insensitive collation