[feature] Mark Multiple Response Header Test Pending - #4640

The Verifies Invoke-WebRequest Supports Multiple response headers with same name test from #4494 is failing on Linux and macOS due to an implementation difference of HttpListener on Linux and macOS where multiple response headers are concatenated instead of sent as separate headers.

In Windows:

HTTP/1.1 200 OK
Content-Length: 2
Content-Type: text/plain
Server: Microsoft-HTTPAPI/2.0
X-Fake-Header: testvalue01
X-Fake-Header: testvalue02
Date: Tue, 22 Aug 2017 09:49:37 GMT

In Linux/macOS:

HTTP/1.1 200 OK
X-Fake-Header: testvalue01, testvalue02
Server: Microsoft-NetCore/2.0
Date: Tue, 22 Aug 2017 09:18:18 GMT
Content-Type: text/plain
Content-Length: 2

This is not a failure of the test but an issue with the current method used for serving test content with HttpListener.

Issue #4639 has been created to track creating a cross-platform multiple header response solution for this test. In the mean time, mark this test as pending to prevent nightly failures.