length.

When temporary field created for DATE(LEFT(column,8)) expression, max_length
value is taken from Item_date_typecast, and it is getting it from underlaid
Item_func_left and it's max_length is 8 in given expression. And all this
results in stripping last 2 digits.

To Item_date_typecast class added its own fix_length_and_dec() function 
that sets max_length value to 10, which is proper for DATE field.


mysql-test/t/group_by.test:
  Test case for bug#12266 GROUP BY DATE(LEFT(column,8)) returns result strings with reduced length.
mysql-test/r/group_by.result:
  Test case for bug#12266 GROUP BY DATE(LEFT(column,8)) returns result strings with reduced length.
sql/item_timefunc.h:
  Fix bug#12266 GROUP BY DATE(LEFT(column,8)) returns result strings with reduced length.
  To Item_date_typecast class added its own fix_length_and_dec() which sets proper max_length value.