Item::tmp_table_field_from_field_type() and create_tmp_field_from_item()
was converting string field to blob depending on byte-wise length instead of
character length, which results in converting valid varchar string with
length == 86 to longtext.

Made that functions above take into account max width of character when
converting string fields to blobs.


sql/item.cc:
  Fix bug #12537 UNION produces longtext instead of varchar
  Item::tmp_table_field_from_field_type() now taking into account max char width when creating tmp field for string fields.
sql/sql_select.cc:
  Fix bug #12537 UNION produces longtext instead of varchar
   create_tmp_field_from_item()now taking into account max char width when creating tmp field for string fields.
mysql-test/r/create.result:
  Test case for bug #12537 UNION produces longtext instead of varchar
mysql-test/t/create.test:
  Test case for bug #12537 UNION produces longtext instead of varchar