search play.plugins descriptor in application path

* it's important when <application dir> is not the same as <working dir>
* in other words, when admin starts the application with a command like `cd /tmp && play start ~/hello`

Author: asolntsev

framework/src/play/plugins/PluginCollection.java

@@ -196,7 +196,7 @@ List<URL> loadPlayPluginDescriptors() {
         try {
             String playPluginsDescriptor = Play.configuration.getProperty("play.plugins.descriptor");
             if (playPluginsDescriptor != null) {
-                return Collections.singletonList(new File(playPluginsDescriptor).toURI().toURL());
+                return Collections.singletonList(new File(Play.applicationPath, playPluginsDescriptor).toURI().toURL());
             }
             return Collections.list(Play.classloader.getResources(play_plugins_resourceName));
         }

framework/test-src/play/plugins/PluginCollectionTest.java

@@ -98,7 +98,7 @@ public void canLoadPlayPluginsFromASingleDescriptor() throws Exception {
         Play.configuration.setProperty("play.plugins.descriptor", "test-src/play/plugins/custom-play.plugins");
         PluginCollection pc = new PluginCollection();
         assertThat(pc.loadPlayPluginDescriptors()).containsExactly(
-                new File("test-src/play/plugins/custom-play.plugins").toURI().toURL()
+                new File(Play.applicationPath, "test-src/play/plugins/custom-play.plugins").toURI().toURL()
         );
     }