for WebRedirect, don't "match" when the destination URL has the same scheme,host,path, for doing http -> https redirects without loops

you can already get most http to https redirects through DontRedirectPlainHTTP
in WebHandler, but that needs handlers for all paths.

now you can just set up a redirect for a domain and all its path to baseurl
https://domain (leaving other webdirect fields empty). when the request comes
in with plain http, the redirect to https is done. that next request will also
evaluate the same redirect rule. but it will not cause a match because it would
redirect to the same scheme,host,path. so next webhandlers get a chance to
serve.

also clarify in webhandlers docs that also account & admin built-in handlers
run first.

related to issue #16

http/webserver_test.go

@@ -50,6 +50,10 @@ func TestWebserver(t *testing.T) {
 
 	test("GET", "http://redir.mox.example", nil, http.StatusPermanentRedirect, "", map[string]string{"Location": "https://mox.example/"})
 
+	// http to https redirect, and stay on https afterwards without redirect loop.
+	test("GET", "http://schemeredir.example", nil, http.StatusPermanentRedirect, "", map[string]string{"Location": "https://schemeredir.example/"})
+	test("GET", "https://schemeredir.example", nil, http.StatusNotFound, "", nil)
+
 	test("GET", "http://mox.example/static/", nil, http.StatusOK, "", map[string]string{"X-Test": "mox"})                              // index.html
 	test("GET", "http://mox.example/static/dir/", nil, http.StatusOK, "", map[string]string{"X-Test": "mox"})                          // listing
 	test("GET", "http://mox.example/static/dir", nil, http.StatusTemporaryRedirect, "", map[string]string{"Location": "/static/dir/"}) // redirect to dir