test: expand test coverage for url.js

Currently, line 156 of lib/url.js is not reachable from test-url because
there is no example URL which has a white space in the front of the url.
I added one example which can reach that line.

PR-URL: https://github.com/nodejs/node/pull/8859
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Franziska Hinkelmann <fhinkel@vt.edu>

test/parallel/test-url.js

@@ -158,6 +158,17 @@ var parseTests = {
     path: '/Y'
   },
 
+  // whitespace in the front
+  ' http://www.example.com/': {
+    href: 'http://www.example.com/',
+    protocol: 'http:',
+    slashes: true,
+    host: 'www.example.com',
+    hostname: 'www.example.com',
+    pathname: '/',
+    path: '/'
+  },
+
   // + not an invalid host character
   // per https://url.spec.whatwg.org/#host-parsing
   'http://x.y.com+a/b/c': {