internal/e2e: prepare the body for POST request before http.Do (#227)
internal/e2e: prepare the body for POST request before http.Do (#227)

As of Go 1.8, NewRequest reads from the body. Instead of writing to the
body's buffer after the call to NewRequest, call NewRequest after
first preparing the multipart body.
GoogleCloudPlatform/golang-samples,danicat/golang-samples,adamyanliev/golang-samples,GoogleCloudPlatform/golang-samples,danicat/golang-samples,adamyanliev/golang-samples,GoogleCloudPlatform/golang-samples,danicat/golang-samples,adamyanliev/golang-samples,adamyanliev/golang-samples