fix bug: I should have checked for definedness rather than for simple…

… true/false

When the first element is the element to be replaced, the index is 0. That
would be 'false', hence the element wasn't replaced.

t/ppi_element_replace.t

@@ -3,11 +3,10 @@
 # Unit testing for PPI::Element
 
 use lib 't/lib';
+use PPI::Document ();
 use PPI::Test::pragmas;
 use Test::More;
 
-use PPI;
-
 __REPLACE: {
 	my $Document = PPI::Document->new( \"print 'Hello World';" );
 	isa_ok( $Document, 'PPI::Document' );