fix end of workday critical bug

Hi,

We noticed some weird behavior when computing business hours in our production app.
I found the culprit : 
```ruby
    BusinessTime::Config.end_of_workday
```
It was returning '23:59:59' when a end of day work hour is '20:00'...

I also added a fixed when computing business hours between ActiveSupport::TimeWithZone

Specs are green on ruby 2.1.1