Fix expect_output_file() on Windows

[krlmlr]

Need to read and write as UTF-8 with LF (krlmlr/enc#8).

[hadley]

Is there any easy fix for this?

[krlmlr]

By now we have read_lines_with_encoding() in this package. We also need a better writing function w.r.t. encoding and line separator. I wonder if the utf8 package is a better place for these functions; I might be able to release utf8 just in time.