Code coverage

Code coverage is almost perfect, Istanbul is reporting:

```
Statements: 88.32% (839 / 950)      
Branches: 74.66% (442 / 592)      
Functions: 95.51% (85 / 89)      
Lines: 90.28% (808 / 895)      
Ignored: none
```

Aim for 100% of code coverage is good thing since it's the "base" of `Protobuf.js` 
