After-fix for WL#2649 Number-to-string conversion.

This change fixes test failure for partition_repair_myisam
and partition_recover_myisam.

The problem was that:
- the file std_data/corrupt_crash.MYI
  contains charsetnr=63 (binary) for index segments.
- the new .frm file contains charsetnr=8 (latin1) for
  the same segments.
As a result REPAIR refused to repair frm+MYI files
with different segment definition.

This fix restores the old behavior for frm and MYI files:
they now store charsetnr=63 for numeric/datetime data types.

storage/heap/ha_heap.cc

@@ -689,7 +689,7 @@ int ha_heap::create(const char *name, TABLE *table_arg,
       if (field->flags & (ENUM_FLAG | SET_FLAG))
         seg->charset= &my_charset_bin;
       else
-        seg->charset= field->charset();
+        seg->charset= field->charset_for_protocol();
       if (field->null_ptr)
       {
 	seg->null_bit= field->null_bit;

storage/myisam/ha_myisam.cc

@@ -274,7 +274,7 @@ int table2myisam(TABLE *table_arg, MI_KEYDEF **keydef_out,
       keydef[i].seg[j].bit_start= keydef[i].seg[j].bit_end=
         keydef[i].seg[j].bit_length= 0;
       keydef[i].seg[j].bit_pos= 0;
-      keydef[i].seg[j].language= field->charset()->number;
+      keydef[i].seg[j].language= field->charset_for_protocol()->number;
 
       if (field->null_ptr)
       {