fix issue: The web_proxy doesn't work when set proxy with uri

[ceasor]

http_client_winhttp::send_request only handle the case when proxy.is_auto_discovery() return true.

So when we construct a web_proxy with uri, it will only set the proxy mode as user_provided_.

So send_request will ignore the proxy setting when we use uri to construct a web_proxy.

My modification tries to fix the problem.

[ceasor]

@microsoft-github-policy-service agree