Bug 6854: Correct a double password encryption problem in import_borrowers.pl

Bug 6854: Correct a double password encryption problem in import_borrowers.pl

When the CSV file has no password column and no default value was set for
the password :
If we try to modify an existing user, the password from the BD is sent
to ModMember, which then encrypts the already-encrypted password a
second time.

http://bugs.koha-community.org/show_bug.cgi?id=6854

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Confirmed bug on current master.
1) Import new patron with password provided
> OPAC login works
2) Overwrite patron record, password provided
> OPAC login works
3) Overwrite patron record, password not provided
Before patch: OPAC login broken, password in database changed
After patch: OPC login works with same password as before

Signed-off-by: Ian Walls <ian.walls@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>