Fix text loader to split only on universal newlines
Currently, `text` loader breaks on a superset of universal newlines, which also contains Unicode line boundaries. See: https://docs.python.org/3/library/stdtypes.html#str.splitlines

However, the expected behavior is to get the lines splitted only on universal newlines: "\n", "\r\n" and "\r".

See: oscar-corpus/corpus#18

Fix #3729.