remove lenreader & add unit tests

[tarunKoyalwar]

Description

• removed LenReader introduced in Adding reusable reader #27 and its dependent

// LenReader is an interface implemented by many in-memory io.Reader's. Used
// for automatically sending the right Content-Length header when possible.

type LenReader interface {
	Len() int
}

io.TeeReader does not follow/implement LenReader hence ReusableReadCloser always fails with LenReader .

• Added Unit Test to test compatiblility

[tarunKoyalwar]

Replacement for LenReader

// getLength returns length of a Reader efficiently
func getLength(x io.Reader) (int64, error) {
	len, err := io.Copy(io.Discard, x)
	return len, err
}

[Mzack9999]

lgtm!