Fix an issue with implied multiplication in the contextFraction.pl macro.

The issue occurs with the following MWE:

```
DOCUMENT();
loadMacros(qw(PGstandard.pl MathObjects.pl contextUnits.pl contextFraction.pl));
Context(context::Units::extending('Fraction')->withUnitsFor('length'));
$a = Compute('2*3 cm');
ENDDOCUMENT();
```

Attempting to open the problem with the develop or main branch will
consume all of your server's resources, and eventually the oomkiller
will kill the process.

The changes in this pull request were suggested by @dpvc in an email
communication between @Alex-Jordan, @dpvc, and myself.

Author: drgrice1

macros/contexts/contextFraction.pl

@@ -646,7 +646,15 @@ sub _check {
 #  operator we didn't otherwise subclass.
 #
 package context::Fraction::BOP::Space;
-our @ISA = ('context::Fraction::BOP::space');
+our @ISA = ('context::Fraction::Class', 'Parser::BOP');
+
+sub _check {
+	my $self    = shift;
+	my $context = $self->context;
+	$self->{bop} = $self->{def}{string};
+	$self->{def} = $context->{operators}{ $self->{bop} };
+	return $self->mutate->_check;
+}
 
 #################################################################################################
 #################################################################################################