Mutations properties are underscore styled

Author: Josema

package.json

@@ -1,6 +1,6 @@
 {
     "name": "dop",
-    "version": "0.28.2",
+    "version": "0.30.0",
     "main": "./dist/dop.nodejs.js",
     "browser": "./dist/dop.js",
     "unpkg": "./dist/dop.min.js",

test/mutation.js

@@ -254,7 +254,7 @@ test('Array.splice', function(t) {
         var mutation = mutations[0]
         t.deepEqual(array.array.length, 4, 'array.array.length')
         t.deepEqual(mutation.object.length, 4, '.object.length')
-        t.deepEqual(mutation.oldLength, 6, '.oldLength')
+        t.deepEqual(mutation.old_length, 6, '.old_length')
         t.equal(mutation.object, array.array, '.object')
         t.equal(mutation.prop, 'array', '.prop')
         t.deepEqual(mutation.splice, [ 3, 3, [ true ] ], '.splice')