Writing lower case numpy dtype field names is a "nono"

Got bit by this and documented it in alpacahq/marketstore#324. I feel that writing a test would be handy for tracking as well as maybe some protection client-side to avoid users hitting this.

2 questions:
- should the client maybe do better error reporting from the server? Right now this is being deferred to user client code. I wonder if a flag like `Client(raise_response_errors=True)` would be handy?
- There should be something in the docs stating that you can't use lower case structarray field names or even make `Client.write()` check for capital field names? This situation makes me think that the `Client.create()` approach (from #37) would allow for this without having to check in every `.write()` call.