Remove strict check for Content-Length header

[faulesocke]

This header is automatically removed from the response by reqwest on
compressed responses (and also does not make any sense there).

This closes #40

[faulesocke]

rustfmt also did some cleanups to the code, do you want those as well?

[jonas-schievink]

Please update the doc comment, too. I think it's best if it doesn't mention the exact checks it performs.

I'd prefer to hold off on running rustfmt for now. I should probably take a look at rustfmt and run it on everything if it makes it look better.

[jonas-schievink]

Thanks!