fix regression: mixin guards compare units correctly

lib/less/tree/dimension.js

@@ -93,7 +93,7 @@ tree.Dimension.prototype = {
             } else if (bValue < aValue) {
                 return 1;
             } else {
-                if (!b.unit.isEmpty() && a.unit.compare(b) !== 0) {
+                if (!b.unit.isEmpty() && a.unit.compare(b.unit) !== 0) {
                     return -1;
                 }
                 return 0;

test/css/mixins-guards.css

@@ -69,3 +69,8 @@
   content: is not theme2;
   content: is theme1 no quotes;
 }
+#tryNumberPx {
+  catch: all;
+  declare: 4;
+  declare: 4px;
+}

test/less/mixins-guards.less

@@ -121,4 +121,17 @@
     .stringguard("theme1");
 	.stringguard("theme2");
 	.stringguard(theme1);
+}
+
+.mixin(...) {
+  catch:all;
+}
+.mixin(@var) when (@var=4) {
+  declare: 4;
+}
+.mixin(@var) when (@var=4px) {
+  declare: 4px;
+}
+#tryNumberPx {
+  .mixin(4px);
 }