Added new method to set string's charset

sql/item_cmpfunc.cc:
  Fix for IN, when String->charset is empty

sql/item_cmpfunc.cc

@@ -890,6 +890,9 @@ void in_string::set(uint pos,Item *item)
   String *res=item->val_str(str);
   if (res && res != str)
     *str= *res;
+  // BAR TODO: I'm not sure this is absolutely correct
+  if (!str->charset())
+      str->set_charset(default_charset_info);
 }
 
 byte *in_string::get_value(Item *item)

sql/sql_string.h

@@ -74,6 +74,7 @@ class String
     { sql_element_free(ptr_arg); }
   ~String() { free(); }
 
+  inline void set_charset(CHARSET_INFO *charset) { str_charset=charset; }
   inline CHARSET_INFO *charset() const { return str_charset; }
   inline uint32 length() const { return str_length;}
   inline uint32 alloced_length() const { return Alloced_length;}