Repository: final-form/react-final-form (js)

Issue:
Pass render prop in Field
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### Are you submitting a **bug report** or a **feature request**?
bug report and documentation improvement request
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### What is the current behavior?
I have a component that uses render prop.
Let's take an example:
```js
const CustomSelect = ({ input, meta, render, children, ...restProps }) => (
  <select {...input} {...restProps}>{render && render()}</select>
);
```
I want to be able to use it with `Field` component like that:
```js
const countries = [
  <option key="placeholder" value="placeholder" disabled>
    Select your country
  </option>,
  <option key="de" value="de">
    Germany
  </option>
];

<Field
  name="country"
  component={CustomSelect}
  render={() => countries}
/>
```

With current `Field` implementation, render prop will *not* be passed down to my `CustomSelect` component. However `children` prop would be passed. Consequently, **we're not able to use form components that use render prop** (this is such an awesome pattern, isn't it?).

The other problem is that this is inconsistent behavior because both `render` and `children` props are part of `Field` API. One of them is being passed down, the other is not.

<!-- If this is a bug, please include steps to reproduce and a minimal demo of the problem using Sandbox, Plunkr, WebpackBin or JSFiddle. -->

### What is the expected behavior?
Both `children` and `render` props should be passed down if `component` is used.
Also, the documentation should explain an order in which `component`, `render`, and `children` are used for rendering as well as what happens when you use more than one of them at the same time.

### Sandbox Link
[![Edit lxnxzr3l19](https://codesandbox.io/static/img/play-codesandbox.svg)](https://codesandbox.io/s/lxnxzr3l19)
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### What's your environment?

<!-- Include 🏁 React Final Form version, 🏁 Final Form version, OS/browser affected, Node version, etc. -->
package | version
---------|---------
React Final Form | 1.1.1
Final Form | 1.2.1

Pull request description:
Pass render prop when component is used
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Fixes #54.

Patch:
diff --git a/src/renderComponent.js b/src/renderComponent.js
index a344614..94cd2c0 100644
--- a/src/renderComponent.js
+++ b/src/renderComponent.js
@@ -11,7 +11,7 @@ export default function renderComponent<T>(
 ): React.Node {
   const { render, children, component, ...rest } = props
   if (component) {
-    return React.createElement(component, { ...rest, children }) // inject children back in
+    return React.createElement(component, { ...rest, children, render })
   }
   if (render) {
     return render({ ...rest, children }) // inject children back in


Test patch:
diff --git a/src/renderComponent.test.js b/src/renderComponent.test.js
new file mode 100644
index 0000000..44d30b1
--- /dev/null
+++ b/src/renderComponent.test.js
@@ -0,0 +1,16 @@
+import renderComponent from './renderComponent'
+
+describe('renderComponent', () => {
+  it('should pass both render and children prop', () => {
+    const children = 'some children'
+    const render = () => 'examplary render function'
+    const props = {
+      component: () => null,
+      children,
+      render
+    }
+    const name = 'TestComponent'
+    const result = renderComponent(props, name)
+    expect(result.props).toEqual({ children, render })
+  })
+})