Add PropType validation for next/head children (#5504)

Give `Head` a more detailed error messages in dev mode by validating the children prop's PropTypes.

packages/next-server/lib/head.js

@@ -100,4 +100,12 @@ function unique () {
   }
 }
 
+if (process.env.NODE_ENV === 'development') {
+  const exact = require('prop-types-exact')
+
+  Head.propTypes = exact({
+    children: PropTypes.oneOfType([PropTypes.element, PropTypes.arrayOf(PropTypes.element)]).isRequired
+  })
+}
+
 export default sideEffect(reduceComponents, onStateChange, mapOnServer)(Head)