Improved control of Content-Type relating to page.render()

[arnholm]

With reference to #310 and page.render() it is observed that page.render() returns a simple string and when used with a CROW_ROUTE's return, it will not properly set the Content-Type header to text/html.

It would probably be worth adding 2 rendering methods, one that returns a string and another that returns a crow::returnable object.