Fix bug with weekday option not working as expected when shift is enabled

[shime]

Hey Nando,

Thank you for the wonderful gem, we have been using it for years and it's really useful.

@pmrazovic has noticed a bug with how it handles weekday option when shift is enabled, so this PR fixes it.
We give users access to specify the rules for recurrence so we generally always want to enable shift.

Without the fix these are the first three Recurrence events for

r = Recurrence.new(every: :month, starts: "2011-01-31", on: "first", weekday: "monday", shift: true)
r.events

2011-02-07 ← first Monday of the month
2011-03-28 ← not the first Monday of the month
2011-09-26 ← not the first Monday of the month

This is with the fix:

2011-02-07 ←  first Monday of the month
2011-03-07 ←  first Monday of the month
2011-04-04 ←  first Monday of the month

[shime]

Not sure why the tests are failing, I've tried updating the to_s(:date) references but that still didn't fix them, although it works locally™.

[fnando]

Closed via #41