Small fix for docx date field format import.

The standard allows for arbitrary amounts of whitespace between
tokens. MSO outputs with a space between \@ and ", LO doesn't.
To account for this, any whitespace between these tokens is
stripped and parsing assumes a command looks like:
DATE \@"some format".

Change-Id: I2620780da3ee873af6f35e236c7c34a073aebe73
Reviewed-on: https://gerrit.libreoffice.org/13430
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>