Change check to detect Windows

(cherry picked from commit 6e7b1a50125207e74ac51b4d76168909280ffa3b)

Change-Id: I39497d77a23d343330dbcc54e5ff361e722bec08
WebRequest.GetSystemWebProxy() throws a NullReferenceException

in case the proxy bypass list is empty or has not been configured
for the system. This is true when running MonoDevelop 2.8.2 in Linux
to debug an ASP.NET application. In this case, the debugger does not
have access to the no_proxy environment variable. As a result,
WebClient cannot be used in such an environment.

Cherry-pick from 485b40e9a29e055305c494c0a941a3e6978dd265.

Change-Id: I3d7e957f4ddbb229528eb9ef15050bec3dfb25e5