click_button: fix count of present exclusive options (use a counter)

with options "x" and/or "y", @present_exclusive_options would contain "undef" elements, which will be counted too. Therefore click_button with x and y will always have more than > 1 options.

lib/WWW/Mechanize.pm

@@ -2524,23 +2524,18 @@ sub click_button {
     my $self = shift;
     my %args = @_;
 
+	my $present_exc_options = 0;
     for ( keys %args ) {
         if ( !/^(number|name|value|id|input|x|y)$/ ) {
             $self->warn(qq{Unknown click_button parameter "$_"});
         }
+        else {
+        	$present_exc_options++ if !/^(?:x|y)$/;
+        }
     }
 
-    my %exclusive_options = (
-        id     => 1,
-        input  => 1,
-        name   => 1,
-        number => 1,
-        value  => 1,
-    );
-
-    my @present_exclusive_options = @exclusive_options{ keys %args };
-
-    if ( scalar @present_exclusive_options > 1 ) {
+    # die if more than one exclusive option is provided
+    if ( $present_exc_options > 1 ) {
         $self->die(
             'click_button: More than one button selector has been used');
     }