Source#current_offset= returns true instead of the set value, causing
Source#done? to blow up on (self.cur_offset ||= start_offset) >= end_offset

The fix is to swap the two lines in Source#current_offset= so that it returns
the value:

  def cur_offset= o
    @dirty = true
    @cur_offset = o
  end

As it is in 0.10.2 it returns the value for @dirty instead