Fixes warnings on destruction of MathObject.

Author: pstaabp

macros/contexts/contextExtensions.pl

@@ -467,7 +467,7 @@ sub extensionContext {
 sub AUTOLOAD {
 	our $AUTOLOAD;
 	my $self = shift;
-	return unless $self;
+	return unless defined $self;
 	my $class  = $self->extensionContext;
 	my $method = (split(/::/, $AUTOLOAD))[-1];
 	return if $method eq 'DESTROY';

macros/contexts/contextSignificantFigures.pl

@@ -18,7 +18,7 @@ =head1 DESCRIPTION
 
 or
 
-    Context('LimitedSignificantFigures');  # TO BE DONE
+    Context('LimitedSignificantFigures');
 
 where the latter context, you or the student are not allowed to perform any operations on
 any numbers.