Verify certificates in client examples:

fix #1237

HTTP client examples now verify the server's certificate
and generate an error if the certificate is invalid or
expired:

* Set certificate verify mode
* Remove duplicate root certificate

example/http/client/async-ssl/http_client_async_ssl.cpp

@@ -225,6 +225,9 @@ int main(int argc, char** argv)
 
     // This holds the root certificate used for verification
     load_root_certificates(ctx);
+    
+    // Verify the remote server's certificate
+    ctx.set_verify_mode(ssl::verify_peer);
 
     // Launch the asynchronous operation
     std::make_shared<session>(ioc, ctx)->run(host, port, target, version);