Fix HTTPS checks in xplat HttpClient

Author: vijaykota

src/System.Net.Http/src/Resources/Strings.resx

@@ -285,4 +285,7 @@
   <data name="net_cookie_attribute" xml:space="preserve">
     <value>The '{0}'='{1}' part of the cookie is invalid.</value>
   </data>
-</root>
+  <data name="net_http_unix_invalid_client_cert_option" xml:space="preserve">
+    <value>libcurl supports only manual client certificate selection</value>
+  </data>
+</root>

src/System.Net.Http/src/System/Net/Http/Unix/CurlHandler.cs

@@ -107,6 +107,22 @@ internal ICredentials Credentials
                 _serverCredentials = value;
             }
         }
+
+        internal ClientCertificateOption ClientCertificateOptions
+        {
+            get
+            {
+                return ClientCertificateOption.Manual;
+            }
+            set
+            {
+                if (ClientCertificateOption.Manual != value)
+                {
+                    throw new PlatformNotSupportedException(SR.net_http_unix_invalid_client_cert_option);
+                }
+            }
+        }
+
         #endregion
 
         protected override void Dispose(bool disposing)
@@ -127,10 +143,6 @@ protected internal override Task<HttpResponseMessage> SendAsync(
             {
                 throw new ArgumentNullException("request", SR.net_http_handler_norequest);
             }
-            if (request.RequestUri.Scheme == UriSchemeHttps)
-            {
-                throw NotImplemented.ByDesignWithMessage("HTTPS stack is not yet implemented");
-            }
             if (request.Content != null)
             {
                 throw NotImplemented.ByDesignWithMessage("HTTP requests with a body are not yet supported");

src/System.Net.Http/src/System/Net/Http/Unix/HttpClientHandler.Unix.cs

@@ -39,8 +39,14 @@ public CookieContainer CookieContainer
 
         public ClientCertificateOption ClientCertificateOptions
         {
-            get { throw NotImplemented.ByDesignWithMessage("HTTP stack not implemented"); }
-            set { throw NotImplemented.ByDesignWithMessage("HTTP stack not implemented"); }
+            get
+            {
+                return _curlHandler.ClientCertificateOptions;
+            }
+            set
+            {
+                _curlHandler.ClientCertificateOptions = value;
+            }
         }
 
         public DecompressionMethods AutomaticDecompression