code: public Postcard withBoolean(@Nullable String key, boolean value) {
          mBundle.putBoolean(key, value);
          return this;
      }
docs: Inserts a Boolean value 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 boolean
  @return current