Schema/Field: add foreign_key_reference builder

foreign_key_reference was only being set from the is_foreign_key
builder.  If a user manually set is_foreign_key, as DBIx::Class does,
foreign_key_reference would never be set, breaking the HTML producer.

lib/SQL/Translator/Schema/Field.pm

@@ -180,6 +180,7 @@ has foreign_key_reference => (
     is => 'rw',
     predicate => '_has_foreign_key_reference',
     isa => schema_obj('Constraint'),
+    builder => 1,
     weak_ref => 1,
 );
 
@@ -197,6 +198,22 @@ around foreign_key_reference => sub {
     $self->$orig;
 };
 
+sub _build_foreign_key_reference {
+    my ( $self ) = @_;
+
+    if ( my $table = $self->table ) {
+        for my $c ( $table->get_constraints ) {
+            if ( $c->type eq FOREIGN_KEY ) {
+                my %fields = map { $_, 1 } $c->fields;
+                if ( $fields{ $self->name } ) {
+                    return $c;
+                }
+            }
+        }
+    }
+    return undef;
+}
+
 =head2 is_auto_increment
 
 Get or set the field's C<is_auto_increment> attribute.