code: public void writeStringField(String fieldName, String value)
          throws IOException, JsonGenerationException
      {
          writeFieldName(fieldName);
          writeString(value);
      }
docs: Convenience method for outputting a field entry ("member")
  that has a String value. Equivalent to:
  <pre>
  writeFieldName(fieldName);
  writeString(value);
  </pre>
  <p>
  Note: many performance-sensitive implementations override this method