null transport should default to http.DefaultTransport

contrib/net/http/roundtripper.go

@@ -82,6 +82,9 @@ func (rt *roundTripper) Unwrap() http.RoundTripper {
 // WrapRoundTripper returns a new RoundTripper which traces all requests sent
 // over the transport.
 func WrapRoundTripper(rt http.RoundTripper, opts ...RoundTripperOption) http.RoundTripper {
+	if rt == nil {
+		rt = http.DefaultTransport
+	}
 	cfg := newRoundTripperConfig()
 	for _, opt := range opts {
 		opt(cfg)

contrib/net/http/roundtripper_test.go

@@ -21,6 +21,18 @@ import (
 	"gopkg.in/DataDog/dd-trace-go.v1/internal/globalconfig"
 )
 
+func TestWrapRoundTripper_allow_nil_transport(t *testing.T) {
+	assert := assert.New(t)
+
+	httpClient := &http.Client{}
+	httpClient.Transport = WrapRoundTripper(httpClient.Transport)
+
+	wrapped, ok := httpClient.Transport.(*roundTripper)
+	assert.True(ok)
+
+	assert.Equal(http.DefaultTransport, wrapped.base)
+}
+
 func TestRoundTripper(t *testing.T) {
 	mt := mocktracer.Start()
 	defer mt.Stop()