Fix #12370. Explicitly coerce bool/undef to integer.

src/effects.js

@@ -540,6 +540,7 @@ function genFx( type, includeWidth ) {
 
 	// if we include width, step value is 1 to do all cssExpand values,
 	// if we don't include width, step value is 2 to skip over Left and Right
+	includeWidth = includeWidth? 1 : 0;
 	for( ; i < 4 ; i += 2 - includeWidth ) {
 		which = cssExpand[ i ];
 		attrs[ "margin" + which ] = attrs[ "padding" + which ] = type;