contrib/net/http nil transport should default to http.DefaultTransport

[dillonstreator]

What does this PR do?

Fixes #1223
If a nil transport is provided to WrapRoundTripper, default to http.DefaultTransport

Motivation

The following program panics at runtime:

package main

import (
	"net/http"

	httptrace "gopkg.in/DataDog/dd-trace-go.v1/contrib/net/http"
)

func main() {

	httpClient := &http.Client{}

	httpClient.Transport = httptrace.WrapRoundTripper(httpClient.Transport)

	_, _ = httpClient.Get("https://www.google.com")

}

The panic comes when httpClient.Get is called even though nil shouldn't be a valid parameter for WrapRoundTripper. In some programs, that panic might not come as quickly as seen above.

This is interesting because go itself does not require a transport to be specific on the http.Client as it falls back to the http.DefaultTransport. As such, it's not uncommon for an http client to be created without explicitly specifying the transport.

Describe how to test/QA your changes

N/A

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.