code: public final IonTextWriterBuilder withCharset(Charset charset)
      {
          IonTextWriterBuilder b = mutable();
          b.setCharset(charset);
          return b;
      }
docs: Declares the charset denoting the output encoding,
  returning a new mutable builder if this is immutable.
  Only ASCII and UTF-8 are supported; applications can use the helper
  constants {@link #ASCII} and {@link #UTF8}.
  
  @param charset may be null, denoting the default of UTF-8.
  
  @return this instance, if mutable;
  otherwise a mutable copy of this instance.
  
  @see #getCharset()
  @see #setCharset(Charset)