Add ErrorTransport (#114)

See https://github.com/hashicorp/go-cleanhttp for example on the importance of not using http.DefaultClient.

transport.go

@@ -115,3 +115,12 @@ func DoerTransport(cl interface {
 }) Transport {
 	return RoundTripFunc(cl.Do)
 }
+
+// ErrorTransport always returns the specified error instead of connecting.
+// It is intended for use in testing
+// or to prevent accidental use of http.DefaultClient.
+func ErrorTransport(err error) Transport {
+	return RoundTripFunc(func(req *http.Request) (*http.Response, error) {
+		return nil, err
+	})
+}

transport_example_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"crypto/md5"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -111,9 +112,8 @@ func ExampleLogTransport() {
 		fmt.Println("Error!", err)
 	}
 	// Works for bad responses too
-	baseTrans = requests.RoundTripFunc(func(req *http.Request) (*http.Response, error) {
-		return nil, fmt.Errorf("can't connect")
-	})
+	baseTrans = requests.ErrorTransport(errors.New("can't connect"))
+
 	trans = requests.LogTransport(baseTrans, logger)
 
 	if err := requests.