Fix: undefined local variable or method `e'

The example of using timezones from the README (https://github.com/icalendar/icalendar#example) was causing the following error :

```
NameError:
  undefined local variable or method `e' for #<Icalendar::Event:0x007fc94f7a35f8>
```

This appears to be a leftover from 6a5bd05c - where the Event was previously passed around as `e`, it should now use implicit `self`.

This commit makes the code use the implicit self, and adds a test case which catches this error, based upon the code in the README.

I was unsure if there was an existing convention for test cases like this, so for the moment I have just placed it near a similar block event case. Happy to move if it should go elsewhere.