Fix passing children through higher-order components

[i-like-robots]

Example:

function Link (props) {
  return <a href={props.url} className={props.className}>{props.children}</a>
}

function FancyLink (props) {
  return <Link {...props} className="Fancyness">{props.children}</Link>
}

function MyFancyLink (props) {
  return <FancyLink url="http://www.google.com">Fancy Google</FancyLink>
}

Output before:

<a href="http://www.google.com" class="Fancyness"></a>

Output after:

<a href="http://www.google.com" class="Fancyness">Fancy Google</a>

[coveralls]

Coverage increased (+0.2%) to 96.053% when pulling b0aec75 on support-children-as-props into 2ae71ef on master.