Correct handling of mysql_enable_utf8mb4

[dveeden]

Depending on where in the DSN mysql_enable_utf8mb4 was specified it would set imp_dbh->enable_utf8mb4 or not. But it would always call mysql_options() to set the charset. This commit makes sure imp_dbh->enable_utf8mb4 is set consistently.

Closes #360

[dveeden]

Tested with this:

#!/bin/perl
use strict;
use v5.36;
use DBI;
use Encode ();

-t(STDOUT) && binmode(STDOUT, ':utf8');
-t(STDERR) && binmode(STDERR, ':utf8');

my @dsns = (
	"DBI:mysql:database=test;host=localhost;mysql_enable_utf8mb4=1",
	"DBI:mysql(mysql_enable_utf8mb4=1):database=test;host=localhost"
);

for my $dsn (@dsns) {
	say "dsn: " . $dsn;
	my $dbh = DBI->connect($dsn, "xxxx", "xxxx");

	print "Connected to MySQL " . $dbh->selectrow_array("SELECT VERSION()") . "\n";
	say "mysql_enable_utf8mb4:" . $dbh->{mysql_enable_utf8mb4};

	my $sql = 'SELECT CHAR(0xC3BC USING utf8mb4), LENGTH(CHAR(0xC3BC USING utf8mb4)), CHAR_LENGTH(CHAR(0xC3BC USING utf8mb4))';	
	my ($char, $byte_length, $char_length) = $dbh->selectrow_array($sql);
	print 'character: ' . $char . "\n";
	print 'is_utf8?: ' . int(Encode::is_utf8($char)) . "\n";
	print 'mysql byte length: ' . $byte_length . "\n";
	print 'mysql char length: ' . $char_length . "\n";
	print 'perl byte length: ' . bytes::length($char) . "\n";
	print 'perl char length: ' . length($char) . "\n";
	$dbh->disconnect();
	print "--------------------\n";
}

Output without this PR:

dsn: DBI:mysql:database=test;host=localhost;mysql_enable_utf8mb4=1
Connected to MySQL 8.1.0
mysql_enable_utf8mb4:0
character: ü
is_utf8?: 0
mysql byte length: 2
mysql char length: 1
perl byte length: 2
perl char length: 2
--------------------
dsn: DBI:mysql(mysql_enable_utf8mb4=1):database=test;host=localhost
Connected to MySQL 8.1.0
mysql_enable_utf8mb4:1
character: ü
is_utf8?: 1
mysql byte length: 2
mysql char length: 1
perl byte length: 2
perl char length: 1
--------------------

And with this PR:

dsn: DBI:mysql:database=test;host=localhost;mysql_enable_utf8mb4=1
Connected to MySQL 8.1.0
mysql_enable_utf8mb4:1
character: ü
is_utf8?: 1
mysql byte length: 2
mysql char length: 1
perl byte length: 2
perl char length: 1
--------------------
dsn: DBI:mysql(mysql_enable_utf8mb4=1):database=test;host=localhost
Connected to MySQL 8.1.0
mysql_enable_utf8mb4:1
character: ü
is_utf8?: 1
mysql byte length: 2
mysql char length: 1
perl byte length: 2
perl char length: 1
--------------------