from stored procedure. 

Problem: we replace all references to local variables in stored procedures     
with NAME_CONST(name, value) logging to the binary log. However, if the
value's collation differs we might get an 'illegal mix of collation'           
error as we don't pass the collation to the function.

Fix: pass the value's collation to NAME_CONST().

Note: actually we should pass to NAME_CONST() the value's derivation as well.
It's impossible without the parser modifying. Now we always set the 
derivation to DERIVATION_IMPLICIT, the same as local variables have.