When an alias is set to a column leading spaces are removed from the alias.
But when this is done on aliases set by user this can lead to confusion.

Now Item::set_name() method issues the warning if leading spaces were removed
from an alias set by user.

New warning message is added.


mysql-test/t/select.test:
  Added test case for bug#10977:No warning issued if a column name is truncated.
mysql-test/r/select.result:
  Added test case for bug#10977:No warning issued if a column name is truncated.
sql/sql_yacc.yy:
  Fixed bug#10977: No warning issued if a column name is truncated
  The is_autogenerated_name flag is set before set_name() method call.
sql/item.cc:
  Fixed bug#10977: No warning issued if a column name is truncated
  Now Item::set_name() method issues the warning if leading spaces were removed
  from an alias set by user.