didn't work as expected: collation_server was set not to xxx,
but to the default collation of character set "yyy".
    
With different argument order it worked as expected:
mysqld --character-set-server=yyy --collation-server=yyy 
    
Fix:
initializate default_collation_name to 0
when processing --character-set-server
only if --collation-server has not been specified
in command line.


mysql-test/r/ctype_ucs2_def.result:
  Adding test cast
mysql-test/t/ctype_ucs2_def-master.opt:
  Adding test case
mysql-test/t/ctype_ucs2_def.test:
  Adding test case
sql/mysqld.cc:
  Don't clear default_collation_name when processing 
  --character-set-server if collation has already
  been specified using --collation-server