Add unit tests for Express Map

Here's a few unit tests for `express-map`.  Coverage is currently at:
```
Statements   : 100% ( 68/68 )
Branches     : 85% ( 34/40 )
Functions    : 100% ( 13/13 )
Lines        : 100% ( 63/63 )
```

I wanted to catch as many edge cases (with `express-annotations`) as possible, so the set-up phase for the tests is fairly large.  Let me know if that needs to be minimized, or if the tests need to become more granular.