Fixed sign handling in BYDAY

[Aridor2006]

The RFC describes weekdaynum as: [[plus / minus] ordwk] weekday.
This commit fixes a bug when the optional sign was missing, by adding a plus, which is required by the orderIndexes.

[thet]

hi, can you also provide some unit tests?
or can you provide some more information about what your pull request is fixing? (some test data...)

altough the tests are plagued by heisenbugs (see: #8), with your patch in line there are more failing than before - so someone has to fix some other tests too.

[Aridor2006]

Hi, I can't really get the tests to work reliably, probably because of that bug. But take for example the "Trimonthly recurrence by day" test:
"RRULE:FREQ=MONTHLY;INTERVAL=3;BYDAY=+3TH"
should be the same as
"RRULE:FREQ=MONTHLY;INTERVAL=3;BYDAY=3TH".

The second one fails though, which i fixed by adding the + when parsing BYDAY.

[thet]

ok, i'm adding tests...
without this fix, the parsed RRULE string above is finally returned to the textarea as:

RRULE:FREQ=MONTHLY;INTERVAL=3;BYDAY=+1TH

which is wrong and should be:

RRULE:FREQ=MONTHLY;INTERVAL=3;BYDAY=+3TH

[thet]

tests and changelog added here:
54fbe90