Tests are missing for histograms

[HDembinski]

Histograms are completely untested, and the current implementation of TH1 methods has bugs.

[HDembinski]

I wrote a little test that I am happy to submit, but it relies on the other PRs that I made, so I would make a separate PR for this new test, after the other PRs are merged.

[HDembinski]

More tests should be added later, but it is a start.

[jpivarski]

I've merged the PRs so we're ready for your tests, which would be gladly appreciated! These gaps in testing are a serious problem. However, it's probably evening where you are, and I want to assign a version number of 0.3.0 so that I can move on to getting uproot ready for it. I think we'll need to put them into 0.3.1.