url: keep auth in `url.resolve()` if host matches

Fixes: https://github.com/nodejs/node/issues/8165

PR-URL: https://github.com/nodejs/node/pull/8215
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/url.js

@@ -780,13 +780,13 @@ Url.prototype.resolveObject = function(relative) {
   if (isRelAbs) {
     // it's absolute.
     if (relative.host || relative.host === '') {
+      if (result.host !== relative.host) result.auth = null;
       result.host = relative.host;
       result.port = relative.port;
-      result.auth = null;
     }
     if (relative.hostname || relative.hostname === '') {
+      if (result.hostname !== relative.hostname) result.auth = null;
       result.hostname = relative.hostname;
-      result.auth = null;
     }
     result.search = relative.search;
     result.query = relative.query;

test/parallel/test-url.js

@@ -1588,6 +1588,9 @@ var relativeTests2 = [
   ['mailto:another.host.com',
    'mailto:user@example.org',
    'mailto:another.host.com'],
+  ['https://example.com/foo',
+   'https://user:password@example.com',
+   'https://user:password@example.com/foo'],
 ];
 relativeTests2.forEach(function(relativeTest) {
   const a = url.resolve(relativeTest[1], relativeTest[0]);