move to .NET Standard 2.0

[colemickens]

Looks like the X509Certificate2 API is slightly different...

[colemickens]

(It seems that .NET Standard 2.0 still doesn't seem to support non-PFX scenarios around x509)

[colemickens]

When running the example, I get this:

Unhandled Exception: System.AggregateException: One or more errors occurred. (The handler does not support client authentication certificates with this combination of libcurl (7.55.1) and its SSL backend ("OpenSSL/1.1.0f").) ---> System.PlatformNotSupportedException: The handler does not support client authentication certificates with this combination of libcurl (7.55.1) and its SSL backend ("OpenSSL/1.1.0f").

[colemickens]

It appears the error I've listed is unrelated to moving it to .NETStandard2 and is rather just a deficiency in corefx on Linux/OSX. If I replace my libcurl4 with one built against openssl1.0.0, then it works. Not sure how common that is among modern distros...

This was my workaround in Arch Linux for anyone that winds up here via Google:

$ pacaur -S libcurl-openssl-1.0.0
$ cd examples/simple
$ LD_LIBRARY_PATH=/usr/lib/libcurl-openssl-1.0.0/:$LD_LIBRARY_PATH dotnet run

Starting Request!
nginx-4217019353-8wb5r

Also, this is probably what I get for not just doing this in a container where there's probably an appropriate version of libcurl.

---

RE the PR:

I think this can be merged.

[krabhishek8260]

nit: netstandard2.0, just to be consistent

[colemickens]

Done. Added another commit that removes all the UTF-8 BOMs I could find. It appears VS Code also did some other small cleanups as I touched those files. Hopefully that's okay to bundle in this PR.

[brendandburns]

LGTM, thanks!