Solve "StackOverflowException" on BackgroundTypeAdapter when the obje…

…ct is of an incompatible type

library/src/main/java/com/pengrad/telegrambot/model/chatbackground/BackgroundTypeUnknown.java

@@ -0,0 +1,25 @@
+package com.pengrad.telegrambot.model.chatbackground;
+
+import java.util.Objects;
+
+public class BackgroundTypeUnknown extends BackgroundType {
+
+    public static final String TYPE = "unknown";
+
+    public BackgroundTypeUnknown() {
+        super(TYPE);
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        return true;
+    }
+
+    @Override
+    public String toString() {
+        return "BackgroundTypeUnknown{" +
+            '}';
+    }
+}

library/src/main/java/com/pengrad/telegrambot/utility/gson/BackgroundTypeAdapter.java

@@ -1,11 +1,7 @@
 package com.pengrad.telegrambot.utility.gson;
 
 import com.google.gson.*;
-import com.pengrad.telegrambot.model.chatbackground.BackgroundType;
-import com.pengrad.telegrambot.model.chatbackground.BackgroundTypeChatTheme;
-import com.pengrad.telegrambot.model.chatbackground.BackgroundTypeFill;
-import com.pengrad.telegrambot.model.chatbackground.BackgroundTypePattern;
-import com.pengrad.telegrambot.model.chatbackground.BackgroundTypeWallpaper;
+import com.pengrad.telegrambot.model.chatbackground.*;
 
 import java.lang.reflect.Type;
 
@@ -26,6 +22,6 @@ public BackgroundType deserialize(JsonElement element, Type type, JsonDeserializ
             return context.deserialize(object, BackgroundTypeChatTheme.class);
         }
 
-        return context.deserialize(object, BackgroundType.class);
+        return new BackgroundTypeUnknown();
     }
 }