Union: use provided property names in default union encode

The union definition may have provided property names in which case the
default ones are not what should be used when encoding.

Fixes #6.

lib/Layout.js

@@ -793,12 +793,12 @@ Union.prototype.encode = function (src, b, offset) {
     if (undefined === offset) {
         offset = 0;
     }
-    var dlo = this.discr_layout,
-        vlo = this.default_layout,
-        discr = src.variant,
-        content = src.content;
+    var dlo = this.layout.fields[0],
+        vlo = this.layout.fields[1],
+        discr = src[dlo.property],
+        content = src[vlo.property];
     if ((undefined === discr) || (undefined === content)) {
-        throw new Error("default union encode must be provided variant and content");
+        throw new Error("default union encode must be provided " + dlo.property + " and " + vlo.property);
     }
     dlo.encode(discr, b, offset);
     vlo.encode(content, b, offset + dlo.span);

test/LayoutTest.js

@@ -528,6 +528,21 @@ suite("Layout", function () {
             assert.equal(un.layout.fields[0].property, 'number');
             assert.equal(un.layout.fields[1].property, 'payload');
         });
+        test("issue#6", function () {
+            var dlo = lo.u8('number'),
+                vlo = new lo.Sequence(lo.u8(), 8, 'payload'),
+                un = new lo.Union(dlo, vlo),
+                b = Buffer("000102030405060708", 'hex'),
+                obj = un.decode(b);
+            assert.equal(obj.number, 0);
+            assert(_.isEqual(obj.payload, [1,2,3,4,5,6,7,8]));
+            var b2 = new Buffer(un.span);
+            un.encode(obj, b2);
+            assert.equal(b2.toString('hex'), b.toString('hex'));
+            var obj2 = { 'variant': obj.number,
+                         'content': obj.payload };
+            assert.throws(function () { un.encode(obj2, b2); });
+        });
     });
     test("fromArray", function () {
         assert.strictEqual(lo.u8().fromArray([1]), undefined);