code: public Postcard withSparseParcelableArray(@Nullable String key, @Nullable SparseArray<? extends Parcelable> value) {
          mBundle.putSparseParcelableArray(key, value);
          return this;
      }
docs: Inserts a SparceArray of Parcelable values into the mapping of this
  Bundle, replacing any existing value for the given key.  Either key
  or value may be null.
  
  @param key   a String, or null
  @param value a SparseArray of Parcelable objects, or null
  @return current