Throw error on empty key in context

[fsteeg]

I think with this we can resolve #141.

The actual issue of producing invalid output from Jena has been fixed in Jena as described by @afs in #141 (comment). This pull request adds throwing an error as mentioned by @ansell in #141 (comment).

[coveralls]

Coverage increased (+0.02%) to 89.027% when pulling f406471 on fsteeg:141-errorOnEmptyKey into 47eaff4 on jsonld-java:master.

[fsteeg]

@ansell @tristan, how do you feel about merging this? Let me know if you have any doubts.

In general, should I assign you (one of you? both?) when I open a pull request? And should I rather push to branches in this repo (not my fork) so that you can push changes into proposed pull requests?

[ansell]

If we need to make changes, we can pull locally and merge. Sorry I haven't had a chance to look at this so far. It looks good to me.