GODRIVER-3373: Use non-default global DefaultTransport for DefaultClient

[zhouselena]

GODRIVER-3373

Summary

NewHTTPClient returns a globally defined DefaultHTTPClient that returns DefaultTransport or updates the transport if it is replaced with a non-default transport.

Background & Motivation

Users can now use non-default transports without panic.

[mongodb-drivers-pr-bot]

API Change Report

No changes found!

[prestonvasquez]

The purpose of this change is to avoid cloning http.DefaultTransport.(*http.Transport) if a user changes the global http.DefaultTransport which would otherwise cause a panic when asserting to clone:

http.DefaultTransport.(*http.Transport).Clone()

panic: interface conversion: http.RoundTripper is *loghttp.Transport, not *http.Transport

goroutine 1 [running]:
go.mongodb.org/mongo-driver/v2/internal/httputil.init()
        /Users/preston.vasquez/Developer/mongo-go-driver/internal/httputil/httputil.go:15 +0x84
exit status 2

The current solution avoids this panic but doesn't set the default transport. You can create a small test in the options package to prove this:

func TestClientHTTPTransport(t *testing.T) {
	http.DefaultTransport = &nonDefaultTransport{}
	got := Client().HTTPClient.Transport 
	
	// Assert that got is a nonDefaultTransport
}

The fix is to use httputil. NewHTTPClient() as the HTTP Client when constructing ClientOptions.