Fix user agent string

It previously was `neo4j-javascript/[object Object]` because of an incorrect
usage of default import. This commit makes it `neo4j-javascript/0.0.0-dev`.
Part after `/` will be changed by the pre-release gulp script.

Author: lutovich

src/v1/driver.js

@@ -45,7 +45,7 @@ class Driver {
    * @param {Object} config
    * @access private
    */
-  constructor(url, userAgent = 'neo4j-javascript/0.0', token = {}, config = {}) {
+  constructor(url, userAgent, token = {}, config = {}) {
     this._url = url;
     this._userAgent = userAgent;
     this._openSessions = {};

src/v1/routing-driver.js

@@ -29,7 +29,7 @@ import Integer from './integer'
  */
 class RoutingDriver extends Driver {
 
-  constructor(url, userAgent = 'neo4j-javascript/0.0', token = {}, config = {}) {
+  constructor(url, userAgent, token = {}, config = {}) {
     super(url, userAgent, token, config);
     this._clusterView = new ClusterView(new RoundRobinArray([url]));
   }

src/version.js

@@ -24,4 +24,4 @@
 // This is set up this way to keep the version in the code in
 // sync with the npm package version, and to allow the build
 // system to control version names at packaging time.
-export default { VERSION : "0.0.0-dev" };
+export default "0.0.0-dev";

test/v1/driver.test.js

@@ -142,6 +142,16 @@ describe('driver', function() {
     driver.session();
   });
 
+  it('should have correct user agent', () => {
+    const directDriver = neo4j.driver("bolt://localhost");
+    expect(directDriver._userAgent).toBe("neo4j-javascript/0.0.0-dev");
+    directDriver.close();
+
+    const routingDriver = neo4j.driver("bolt+routing://localhost");
+    expect(routingDriver._userAgent).toBe("neo4j-javascript/0.0.0-dev");
+    routingDriver.close();
+  });
+
   var exposedTypes = [
     'Node',
     'Path',