Refactor: bytes.NewBufferString

[mattes]

Replace

ioutil.NopCloser(bytes.NewBufferString())

with

ioutil.NopCloser(strings.NewReader(s string))

... it's being used in the tests.

https://golang.org/pkg/strings/#NewReader

NewReader returns a new Reader reading from s. It is similar to bytes.NewBufferString but more efficient and read-only.