add function Handler for convenience

[kr]

This function signature makes it easier to do the most
common usage patterns I'm aware of, after several years
of seeing this package used in the wild.

http.Handle("/", secureheader.Handler(mux))
http.ListenAndServe(addr, nil)

and

http.Handle("/a", a)
http.Handle("/b", b)
http.ListenAndServe(addr, secureheader.Handler(nil))

and

var h http.Handler
h = mux
h = middleware3(h)
h = secureheader.Handler(h)
h = middleware1(h)
http.ListenAndServe(addr, h)

[kr]

PTAL

[croaky]

LGTM

[croaky]

Articles like this mention http.DefaultServeMux as a potential security gotcha. Would it be better to use something else here in the context of a security-focused library?

[kr]

Yeah maybe. For the purpose of this PR, I just wanted to update the form of the examples, but not change their behavior. Opened #15 to track this.

[ryandotsmith]

🛋 comfy.
lgtm