Enable the auto parameter of the flag `default-character-set`

Closes #739

When invoking option `--default-character-set=auto` character set
from underlying OS settings should be detected for mysqldump.

client/mysqldump.c

@@ -1057,7 +1057,7 @@ static int get_options(int *argc, char ***argv)
             my_progname_short);
     return(EX_USAGE);
   }
-  if (strcmp(default_charset, charset_info->csname) &&
+  if (strcmp(default_charset, MYSQL_AUTODETECT_CHARSET_NAME) &&
       !(charset_info= get_charset_by_csname(default_charset,
                                             MY_CS_PRIMARY, MYF(MY_WME))))
     exit(1);
@@ -1522,6 +1522,9 @@ static int switch_character_set_results(MYSQL *mysql, const char *cs_name)
   char query_buffer[QUERY_LENGTH];
   size_t query_length;
 
+  if (!strcmp(cs_name, MYSQL_AUTODETECT_CHARSET_NAME))
+    cs_name= (char *)my_default_csname();
+
   /* Server lacks facility.  This is not an error, by arbitrary decision . */
   if (!server_supports_switching_charsets)
     return FALSE;