files.add*Stream coverage is poor

[alanshaw]

Pulling out tests per command for #290 shows that test coverage for addReadableStream and addPullStream is poor (almost non existent).

Considering files.add is basically most developer's first port of call when starting with IPFS these commands should be rock solid and tested as such.

[achingbrain]

This code has mostly been deleted with the async/await refactor, so I don't think there's much to do here now - please open an issue on ipfs/js-ipfs if that's not the case.