The Item::tmp_table_field_from_field_type() function creates Field_datetime
object instead of Field_timestamp object for timestamp field thus always
changing data type is a tmp table is used.

The Field_blob object constructor which is used in the 
Item::tmp_table_field_from_field_type() is always setting packlength field of
newly created blob to 4. This leads to changing fields data type for example
from the blob to the longblob if a temporary table is used.

The Item::make_string_field() function always converts Field_string objects 
to Field_varstring objects. This leads to changing data type from the 
char/binary to varchar/varbinary.

Added appropriate Field_timestamp object constructor for using in the 
Item::tmp_table_field_from_field_type() function.

Added Field_blob object constructor which sets pack length according to
max_length argument.

The Item::tmp_table_field_from_field_type() function now creates
Field_timestamp object for a timestamp field.

The Item_type_holder::display_length() now returns correct NULL length NULL
length. 

The Item::make_string_field() function now doesn't change Field_string to
Field_varstring in the case of Item_type_holder. 

The Item::tmp_table_field_from_field_type() function now uses the Field_blob
constructor which sets packlength according to max_length.


mysql-test/t/union.test:
  Added test case for bug#12185: Data type aggregation may produce wrong result
  Corrected test case after fix for bug#12185
mysql-test/t/innodb.test:
  Corrected test case after fix for bug#12185
mysql-test/r/union.result:
  Added test case for bug#12185: Data type aggregation may produce wrong result
   Corrected test case after fix for bug#12185
mysql-test/r/innodb.result:
  Corrected test case after fix for bug#12185
mysql-test/r/create.result:
  Corrected the test case after fixing bug#12185
sql/field.h:
  Fixed bug#12185: Data type aggregation may produce wrong result
  Added Field_blob object constructor which sets packlength according to
  max_length argument.
sql/item.cc:
  Fixed bug#12185: Data type aggregation may produce wrong result
  The Item::make_string_field() function now doesn't change Field_string to
  Field_varstring in the case of Item_type_holder.
  The Item::tmp_table_field_from_field_type() function now creates
  Field_timestamp object for a timestamp field.
  The Item::tmp_table_field_from_field_type() function now uses the Field_blob
  constructor which sets packlength according to max_length.
  The Item_type_holder::display_length() now returns correct NULL length NULL
  length.
sql/field.cc:
  Fixed bug#12185: Data type aggregation may produce wrong result
  Added appropriate Field_timestamp object constructor for using in the 
  Item::tmp_table_field_from_field_type() function.