fix indexing for reading rational or srational arrays

[sam6321]

Fix for #437

[constantinius]

Hi @sam6321

Thanks for providing the fix. It would be great if you could also add a testcase for that to demonstrate it works.

[sam6321]

Sure I can look at doing that

[sam6321]

@constantinius I tried to add the most minimal test case I could. I'm not sure how to add test files to the repository so I just put the file data inline in the code (it's not really that big so I hope that's okay).

I can instead provide the tiff file directly if that's more preferable