Fix null-pointer vulnerability and restore Android compatibility

The loop checking the list of input extensions passed as a constructor
parameter was blindly calling getClass() on any element in the list.
This is not null safe and broke compatibility with some Android
versions.

Change the code to a null-safe instanceof check, which also restores
Android compatibility.

Fixes Issue #1353.

Author: robert-s-ubi

src/main/java/org/java_websocket/drafts/Draft_6455.java

@@ -244,7 +244,7 @@ public Draft_6455(List<IExtension> inputExtensions, List<IProtocol> inputProtoco
     boolean hasDefault = false;
     byteBufferList = new ArrayList<>();
     for (IExtension inputExtension : inputExtensions) {
-      if (inputExtension.getClass().equals(DefaultExtension.class)) {
+      if (inputExtension instanceof DefaultExtension) {
         hasDefault = true;
       }
     }