Support parsing of headers in any order

[kristopher-h]

When parsing the Base Language Server Protocol allow headers in any order. Currently this is only relevant for the Content-Length header, since that is the only header of interest.

[palantirtech]

Thanks for your interest in palantir/python-jsonrpc-server, @kristopher-h! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.