Add compatibility with Node v0.12

Node v0.12 doesn't have the Object.assign function

lib/sql.js

@@ -5,6 +5,7 @@ var Connector = require('./connector');
 var debug = require('debug')('loopback:connector:sql');
 var ParameterizedSQL = require('./parameterized-sql');
 var Transaction = require('./transaction');
+var assign = require('./utils').assign;
 
 module.exports = SQLConnector;
 
@@ -1067,10 +1068,10 @@ SQLConnector.prototype.buildJoins = function(model, where) {
     var modelToEscaped = this.tableEscaped(modelTo);
     var keyTo = rel.keyTo;
 
-    var innerWhere = Object.assign({}, where[key]);
+    var innerWhere = assign({}, where[key]);
     var innerIdField = {};
     innerIdField[keyTo] = true;
-    innerWhere.fields = Object.assign({}, innerWhere.fields, innerIdField);
+    innerWhere.fields = assign({}, innerWhere.fields, innerIdField);
 
     var condition = this.columnEscaped(model, keyFrom) + '=' +
       this.columnEscaped(modelTo, keyTo);

lib/utils.js

@@ -0,0 +1,18 @@
+var _hasOwnProp = Object.prototype.hasOwnProperty;
+
+/**
+ * Object.assign polyfill
+ */
+var assign = Object.assign || function(target) {
+  for (var i = 1; i < arguments.length; i++) {
+    var source = arguments[i];
+    for (var key in source) {
+      if (_hasOwnProp.call(source, key)) {
+        target[key] = source[key];
+      }
+    }
+  }
+  return target;
+};
+
+exports.assign = assign;