Handle numeric keys for ETF maps (#2642)

src/main/java/net/dv8tion/jda/api/utils/data/etf/ExTermDecoder.java

@@ -278,7 +278,8 @@ private static Map<String, Object> unpackMap0(ByteBuffer buffer)
         int arity = buffer.getInt();
         while (arity-- > 0)
         {
-            String key = (String) unpack0(buffer);
+            Object rawKey = unpack0(buffer);
+            String key = String.valueOf(rawKey);
             Object value = unpack0(buffer);
             map.put(key, value);
         }