SDK: GetTdfType does not reset the reader

[jrschumacher]

I'm not sure if this is expected behavior, but when using the function sdk.GetTdfType(ec) I found that I needed to reset the *io.Reader to use it for decrypting data.

b := &bytes.Buffer{}
pt := io.Writer(b)
ec := bytes.NewReader(toDecrypt)
tdfType := sdk.GetTdfType(ec)
// reset the reader to the beginning
ec.Reset(toDecrypt)
if tdfType == sdk.Nano {
	err := h.sdk.ReadNanoTDF(pt, ec)
	return b, err
}
return nil, errors.New("unsupported TDF type")