Amerdrix/TimeSpan — pull request #1 by osiro (2014-03-18 04:52:20+00:00)
Title: Fix TimeSpan initialize

TimeSpan initialisation works fine when duration is nil and unit is seconds:

```ruby
TimeSpan(nil)
=> 00:00
```

But raise an exception when duration is nil and unit is hours or minutes:

```ruby
TimeSpan(nil, :minutes)
NoMethodError: undefined method `*' for nil:NilClass
```