Fixes bug where double slashes isn't parsed as host. (#599)

* Added test for parsing double slashes as host

* Parse double slashes as host

Author: iifeoluwa

src/utils/urlJoin.js

@@ -6,7 +6,7 @@ const path = require('path');
  * e.g. from \path\to\res.js to /path/to/res.js.
  */
 module.exports = function(publicURL, assetPath) {
-  const url = URL.parse(publicURL);
+  const url = URL.parse(publicURL, false, true);
   url.pathname = path.posix.join(url.pathname, URL.parse(assetPath).pathname);
   return URL.format(url);
 };

test/url-join.js

@@ -64,4 +64,11 @@ describe('Url Join', () => {
   it('should support windows paths', () => {
     assert.equal(urlJoin('dist\\foo', '\\bar\\foo.js'), 'dist/foo/bar/foo.js');
   });
+
+  it('should parse double slashes as host', () => {
+    assert.equal(
+      urlJoin('//parceljs.org/foo?a=123&b=456#hello', 'bar/a.js'),
+      '//parceljs.org/foo/bar/a.js?a=123&b=456#hello'
+    );
+  });
 });