Make the onChange prop not required

The component works fine when no onChange handler is provided, because it creates an input component with the current value which is submitted with forms.

Author: ahuth

src/recaptcha.js

@@ -118,7 +118,7 @@ export default class ReCAPTCHA extends React.Component {
 ReCAPTCHA.displayName = "ReCAPTCHA";
 ReCAPTCHA.propTypes = {
   sitekey: PropTypes.string.isRequired,
-  onChange: PropTypes.func.isRequired,
+  onChange: PropTypes.func,
   grecaptcha: PropTypes.object,
   theme: PropTypes.oneOf(["dark", "light"]),
   type: PropTypes.oneOf(["image", "audio"]),
@@ -130,6 +130,7 @@ ReCAPTCHA.propTypes = {
   badge: PropTypes.oneOf(["bottomright", "bottomleft", "inline"]),
 };
 ReCAPTCHA.defaultProps = {
+  onChange: () => {},
   theme: "light",
   type: "image",
   tabindex: 0,