independently from the expression it is initialized from.
In other words, this change treats a user variable like
a table with one column and one record. Discussed with 
PeterG, Serg and Lars. This change also simplifies replication
allowing not to replicate variables' coercibility.


mysql-test/r/user_var.result:
  Test changes accordintly
mysql-test/t/user_var.test:
  Test changes accordintly