Fix wrong exception when using external auth source

Author: alcaeus

php_phongo.c

@@ -1426,7 +1426,7 @@ static bool php_phongo_uri_finalize_auth(mongoc_uri_t* uri) /* {{{ */
 			}
 		}
 	} else if (require_auth) {
-		if (!username || strcmp(username, "") == 0) {
+		if (source && strcmp(source, "$external") != 0 && (!username || strcmp(username, "") == 0)) {
 			phongo_throw_exception(PHONGO_ERROR_INVALID_ARGUMENT, "Failed to parse URI options: Default authentication mechanism requires username.");
 			return false;
 		}

tests/manager/manager-ctor-auth_source-001.phpt

@@ -0,0 +1,23 @@
+--TEST--
+MongoDB\Driver\Manager::__construct(): authSource option
+--FILE--
+<?php
+
+$tests = [
+    ['mongodb://[email protected]/?authSource=$external', []],
+    [null, ['authSource' => '$external']],
+];
+
+foreach ($tests as $test) {
+    list($uri, $options) = $test;
+
+    /* Note: the Manager's debug information does not include the auth mechanism
+     * so we are merely testing that no exception is thrown. */
+    $manager = new MongoDB\Driver\Manager($uri, $options);
+}
+
+?>
+===DONE===
+<?php exit(0); ?>
+--EXPECT--
+===DONE===