during PO.parse, an extract(string) method is called on each string to
unescape some characters (like " and \). This process should be reverted
in the toString method.
The PO spec says, that all strings should be C-Strings. Otherwise tools
like msgmerge (from the gettext package) will fail parsing po files written
by this library.
I added a few more edge-cases to the tests for the new msgctxt field and
revealed a bug during that. The default value has been set to empty
string, but should have been null, since the spec says, this field is
optional.
