Steps to reproduce

Trying to write a PowerShell stub for https://github.com/ouspg/trytls , I discovered
that PowerShell (running on MacOS 10.11.6) Invoke-Webrequests accepts some invalid
certificates. Additionally, it allows the use of RC4 cryptography, which is now considered insecure.

On Ubuntu 16.04 (after installing some libcurl dependencies), these are rejected.

https://github.com/wbond/badtls.io has additional tests that I could not run, since I could not
figure out how to use a custom CA bundle, these should be investigated as well.

Expected behavior

Invoke-Webrequest https://incomplete-chain.badssl.com/ should fail
Invoke-Webrequest https://rc4.badssl.com/ should fail

Actual behavior

Site loads without throwing exception

## Environment data

Name                           Value

---

PSVersion                      6.0.0-alpha
PSEdition                      Core
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   3.0.0.0
GitCommitId                    v6.0.0-alpha.9
CLRVersion
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1