mysql-test/r/type_varchar.result:
  Update results
mysql-test/t/type_varchar.test:
  Add new regression test
sql/field.cc:
  Add Field::type_can_have_key_part() static method
sql/field.h:
  Add Field::type_can_have_key_part() signature.
sql/sql_table.cc:
  Only reset the length of a key part when changing from a
  field type that can't be used partially to a field that can,
  or vice versa, or when the part is smaller than the length
  of the field.