structure buffer).

This is a follow-up for WL#4435. The bug actually existed not only
MYSQL_TYPE_DATETIME type. The problem was that Item_param::set_value()
was written in an assumption that it's working with expressions, i.e.
with basic data types.

There are two different quick fixes here:
  a) Change Item_param::make_field() -- remove setting of
     Send_field::length, Send_field::charsetnr, Send_field::flags and
     Send_field::type.

     That would lead to marshalling all data using basic types to the client
     (MYSQL_TYPE_LONGLONG, MYSQL_TYPE_DOUBLE, MYSQL_TYPE_STRING and
     MYSQL_TYPE_NEWDECIMAL). In particular, that means, DATETIME would be
     sent as MYSQL_TYPE_STRING, TINYINT -- as MYSQL_TYPE_LONGLONG, etc.

     That could be Ok for the client, because the client library does
     reverse conversion automatically (the client program would see DATETIME
     as MYSQL_TIME object). However, there is a problem with metadata --
     the metadata would be wrong (misleading): it would say that DATETIME is
     marshaled as MYSQL_TYPE_DATETIME, not as MYSQL_TYPE_STRING.

  b) Set Item_param::param_type properly to actual underlying field type.
     That would lead to double conversion inside the server: for example,
     MYSQL_TIME-object would be converted into STRING-object
     (in Item_param::set_value()), and then converted back to MYSQL_TIME-object
     (in Item_param::send()).

     The data however would be marshalled more properly, and also metadata would
     be correct.

This patch implements b).

There is also a possibility to avoid double conversion either by clonning
the data field, or by storing a reference to it and using it on Item::send()
time. That requires more work and might be done later.