FIXED Incorrect timezone_type for DateTime toBeginningOfWeek() #68

This is my first ever contribution to an open source project :) I have been look for a project to contribute to for a while and would love to participate in this one.

Will apologies for anything silly I do in advance.

Commit f71ba5b fixes issue #68. I can't see other commits in have been pulled in to put not sure how to just request pulling f71ba5b. Any advise appreciated.