Use UTF-8 encoding when writing files.

[ahf]

This patch forces write_file_if_contents_differ/2 to use UTF-8 as
encoding when writing files. This fixes an issue where UTF-8 characters
are processed and written back as ISO-8859-1 into the file, which makes
it impossible to use UTF-8 characters in .app.src files.

[ferd]

Does it work better in raw mode rather than {encoding, utf8}? My worry is that we could be reading non-unicode (or in fact, non-text) files and corrupting them here. Using raw mode should bypass encodings set by the IO protocol (defaulting to latin1) and just go, I hope, to byte-direct output.

[ferd]

@ahf would you like me to take over this one? I think I recall an IRC discussion where you were a bit busy to update this PR?

[tsloughter]

Handled by #757