code: public Postcard withCharSequence(@Nullable String key, @Nullable CharSequence value) {
          mBundle.putCharSequence(key, value);
          return this;
      }
docs: Inserts a CharSequence 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 CharSequence, or null
  @return current