Fix GetClientUserAgentString()

[ob-stripe]

Tentative fix for #990.

[jaymedavis]

Almost sure I tried this, but can’t remember entirely. Maybe it worked but didn’t provide the right information? As long as it runs on Mac too, it should be fine!

[ob-stripe]

After a lot of efforts, I was able to run the XUnit test suite using Visual Studio for Mac and .NET Framework 4.6! AFAICT it works fine.

For reference, here are the differences in the X-Stripe-Client-User-Agent string:

• OS X, target net46 (from Visual Studio for Mac):

"lang_version": ".NET Framework 4.5+",
"os_version": "Unix 17.4.0.0",
"mono_version": "5.8.1.0 (2017-10/6bf3922f3fd Thu Mar  8 16:47:23 EST 2018)"

• OS X, target netcoreapp1.1:

"lang_version": ".NET Core 4.6.26011.01",
"os_version": "Darwin 17.4.0 Darwin Kernel Version 17.4.0: Sun Dec 17 09:19:54 PST 2017; root:xnu-4570.41.2~1/RELEASE_X86_64"

• Windows, target net46 (from Appveyor):

"lang_version": ".NET Framework 4.5+",
"os_version": "Microsoft Windows 10.0.14393 "

• Windows, target netcoreapp1.1 (from Appveyor):

"lang_version": ".NET Core 4.6.25211.01",
"os_version": "Microsoft Windows 10.0.14393 "

I've removed the registry lookup entirely -- while it was possible to simply add a check for non-Windows platform, I'm fairly sure the registry lookup was ~useless because it was not returning the current runtime, but rather the most recent framework version available on the machine. (E.g. on Appveyor, it reported "4.7.1 or later" even when targeting 4.6.)

As far as I can tell, there's no good way of checking the actual .NET Framework version at runtime -- there's System.Environment.Version, but it only lets you distinguish between 4.5 and 4.6, and Microsoft actually recommends using the registry instead. At that point I threw my hands in the air and decided "4.5+" was good enough.

r? @brandur-stripe
cc @stripe/api-libraries

[brandur-stripe]

Wow, you are a hero for tracking this one down @ob-stripe! Thanks.

I'll admit up front that I've never looked at these internals before, but this diff and your rationale look correct. I can't believe how difficult it is to do this!

LGTM.