refactor: revert change to disallow authSource without username

Author: mbroadst

lib/core/uri_parser.js

@@ -335,10 +335,6 @@ function applyAuthExpectations(parsed) {
   const options = parsed.options;
   const authSource = options.authsource || options.authSource;
   if (authSource != null) {
-    if (parsed.auth == null || parsed.auth.username == null) {
-      throw new MongoParseError('Auth source provided without username');
-    }
-
     parsed.auth = Object.assign({}, parsed.auth, { db: authSource });
   }
 

test/spec/auth/connection-string.json

@@ -351,11 +351,6 @@
       "valid": true,
       "credential": null
     },
-    {
-      "description": "authSource without username is invalid (default mechanism)",
-      "uri": "mongodb://localhost/?authSource=foo",
-      "valid": false
-    },
     {
       "description": "should throw an exception if no username provided (userinfo implies default mechanism)",
       "uri": "mongodb://@localhost.com/",

test/spec/auth/connection-string.yml

@@ -287,10 +287,6 @@ tests:
         uri: "mongodb://localhost/foo"
         valid: true
         credential: ~
-    -
-        description: "authSource without username is invalid (default mechanism)"
-        uri: "mongodb://localhost/?authSource=foo"
-        valid: false
     -
         description: "should throw an exception if no username provided (userinfo implies default mechanism)"
         uri: "mongodb://@localhost.com/"