Make set_time_zone about 50x faster when the time zone is provided as a string and hasn't changed

set_time_zone already contains an optimization when the time zone is provided as an object, and is the same time zone the DateTime object already has. 

This patch adds an additional check for sameness when the time zone provided is a string. The benchmark below shows is to be about 50x faster for the case it covers.

    use Benchmark ':all';
    use lib './lib';
    use DateTime;
    use DateTime::TimeZone;
    
    my $chicago_tz = DateTime::TimeZone->new( name => 'America/Chicago' );
    my $dt = DateTime->now( time_zone => $chicago_tz );
    
    timethis (100000, sub {
       $dt->set_time_zone('America/Chicago');
       # Reset the object for the next test
    });
    
    # before fix: timethis 100000:  8 wallclock secs ( 7.70 usr +  0.00 sys =  7.70 CPU) @ 12987.01/s (n=100000)
    # after fix: timethis 100000:  0 wallclock secs ( 0.14 usr +  0.00 sys =  0.14 CPU) @ 714285.71/s (n=100000)