Fixed returning bug with oracle

Fixes non working returning() with oracle introduced with #580 (strong-oracle support)

lib/dialects/oracle/index.js

@@ -7,8 +7,6 @@ var _ = require('lodash');
 var Client = require('../../client');
 var Promise = require('../../promise');
 
-var oracle;
-
 // Always initialize with the "QueryBuilder" and "QueryCompiler"
 // objects, which extend the base 'lib/query/builder' and
 // 'lib/query/compiler', respectively.
@@ -131,11 +129,13 @@ Client_Oracle.prototype.preprocessBindings = function(bindings) {
   if (!bindings) {
     return bindings;
   }
-
+
+  var driver = this.driver;
+
   return bindings.map(function(binding) {
-    if (binding instanceof ReturningHelper && oracle) {
+    if (binding instanceof ReturningHelper && driver) {
       // returning helper uses always ROWID as string
-      return new oracle.OutParam(oracle.OCCISTRING);
+      return new driver.OutParam(driver.OCCISTRING);
     }
 
     if (typeof binding === 'boolean') {