COLUMNS ARE USED INSIDE A STORED PROCEDURE                                      
                                                                                
Problem: When 'SET' type columns are used in a DML                              
inside a stored procedure and a NULL value is passed                            
to that column, replication is breaking.                                        
                                                                                
Analysis: All stored procedure variables used inside                            
a DML will be substituted with NAME_CONST functions.                            
While NAME_CONST are used in this particular scenario,                          
i.e., when NULL value is passed then charset is copied                          
from 'empty_set_string' member of Field_set class.                              
The operator '=' overload method inside 'String' class                          
is not coping str_charset from R.H.S object to L.H.S object.                    
Hence charset is wrongly copied in the string assignment                        
                                                                                
Fix: Handle coping str_charset member in operator '=' overload                  
method.

sql/sql_string.h:
  Handled coping str_charset member in operator '=' overload                  
  method.