httplibp2p: cannot set a custom ServeMux

[aschmahmann]

The libp2phttp host's ServeMux is supposed to be modifiable by the user

go-libp2p/p2p/http/libp2phttp.go

Lines 126 to 130 in 87a8d4e

	// ServeMux is the http.ServeMux used by the server to serve requests. If
	// nil, a new serve mux will be created. Users may manually add handlers to
	// this mux instead of using `SetHTTPHandler`, but if they do, they should
	// also update the WellKnownHandler's protocol mapping.
	ServeMux *http.ServeMux

Unfortunately, without some hackery this is difficult to do because creating a custom ServeMux will result in that ServeMux getting clobbered by

go-libp2p/p2p/http/libp2phttp.go

Lines 182 to 186 in 87a8d4e

	func (h *Host) serveMuxInit() {
	h.initializeServeMux.Do(func() {
	h.ServeMux = http.NewServeMux()
	})
	}

that's called in functions like

go-libp2p/p2p/http/libp2phttp.go

Line 261 in 87a8d4e

func (h *Host) Serve() error {

.

Seems like this is most easily fixable by adding a guard to check if the *ServeMux is nil before initializing it