PO Parser should allow things like msgstr "" "foo" in the same line

At the moment, our parser will reject message with this schema:

msgid "Foo"
msgstr "" "Bar"

Gettext accepts it as a good message so we should do the same.

The error we get is:

Traceback (most recent call last):
  File "/srv/launchpad.net/production/launchpad/cronscripts/../lib/canonical/launchpad/database/pofile.py", line 612, in importFromQueue
    entry_to_import.is_published)
  File "/srv/launchpad.net/production/launchpad/cronscripts/../lib/canonical/launchpad/components/poimport.py", line 79, in import_po
    parser.write(file.read())
  File "/srv/launchpad.net/production/launchpad/cronscripts/../lib/canonical/launchpad/components/poparser.py", line 655, in write
    self.parse_line(line)
  File "/srv/launchpad.net/production/launchpad/cronscripts/../lib/canonical/launchpad/components/poparser.py", line 881, in parse_line
    l = self._parse_quoted_string(l)
  File "/srv/launchpad.net/production/launchpad/cronscripts/../lib/canonical/launchpad/components/poparser.py", line 786, in _parse_quoted_string
    "extra content found after string")
POSyntaxError: extra content found after string