Bug/38 context value not mandatory

### Description
ContextAttribute::contextValue is not MANDATORY in FI-WARE NGSI, only in OMA NGSI.
This is fixed in this short PR. Issue #38 can be closed after merging this PR.

If this is a case of mandatory in OMA but optional due to one of out proposed amedments, I'd suggest to use the same text we have used in other places (https://github.com/telefonicaid/fiware-orion/pull/467/files#diff-53fcdc7f4cee2a4514561092a3f8e76fR43):

```
// Optional (FI-WARE changes - MANDATORY in OMA spec)
```