mysqld --collation-server=xxx --character-set-server=yyy

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

mysql-test/r/ctype_ucs2_def.result

@@ -1,3 +1,6 @@
+show variables like 'collation_server';
+Variable_name	Value
+collation_server	ucs2_unicode_ci
 show variables like "%character_set_ser%";
 Variable_name	Value
 character_set_server	ucs2

mysql-test/t/ctype_ucs2_def-master.opt

@@ -1 +1 @@
---default-character-set=ucs2 --default-collation=ucs2_unicode_ci
+--default-collation=ucs2_unicode_ci --default-character-set=ucs2

mysql-test/t/ctype_ucs2_def.test

@@ -1,3 +1,8 @@
+#
+# MySQL Bug#15276: MySQL ignores collation-server
+#
+show variables like 'collation_server';
+
 #
 # Bug#18004 Connecting crashes server when default charset is UCS2
 #