code: public OkRequest<T> form(final String name, final String value, String charset) {
          final boolean first = !mForm;
          if (first) {
              contentType(PROTOCOL_CONTENT_TYPE_FORM);
              mForm = true;
  
          }
          charset = getValidCharset(charset);
  
          openOutput();
          if (!first) {
              mOutput.write('&');
          }
          try {
              if (VolleyLog.DEBUG) {
                  VolleyLog.d("name=%1$s, value=%2$s", name, value);
              }
              mOutput.write(URLEncoder.encode(name, charset));
              mOutput.write("=");
              if (value != null) {
                  mOutput.write(URLEncoder.encode(value, charset));
              }
          } catch (IOException e) {
              //Do Nothing
          }
  
  
          return this;
      }
docs: Write the name/value pair as form data to the request body
  <p/>
  The values specified will be URL-encoded and sent with the
  'application/x-www-form-urlencoded' content-type
  
  @param name
  @param value
  @param charset
  @return this request