Exercise 3: A Real Response.html() Classmethod — Possible Solution
====================================================================

THE CLASSMETHOD
------------------------------
    @classmethod
    def html(cls, markup, status=200):
        return cls(body=markup, status=status, content_type='text/html')

    # registered like any other handler:
    router.add_route('GET', '/', lambda req: Response.html('<h1>Hello</h1>'), name='home')

VERIFIED AGAINST A REAL LIVE REQUEST
------------------------------
    status:       200
    Content-type: text/html
    body:         b'<h1>Hello</h1>'

Content-type reads back as exactly 'text/html', not the class's own
default 'text/plain' - confirmed from the real response object a live
client received, not just from inspecting the Response instance
locally.

WHY THIS WORKS AS AN ANSWER
------------------------------
It follows the exact same shape as Response.json() from the chapter
itself - a classmethod that builds and returns a new Response with one
field (content_type) pinned to a specific value - reusing to_wsgi()'s
own existing Content-Length computation and header-list construction
completely unchanged, and confirms the real header value a client
actually receives rather than only checking the Response object's own
attribute before it's ever sent.