fix(proxy): send Proxy-Authorization when username has no password

httpProxyDialer only emitted the Basic credential header when the
url.Userinfo had a password component set. That dropped auth for
two legitimate cases allowed by RFC 7617: a proxy URL like
http://user@proxy/ (no colon, password unset), and an explicit
empty password http://user:@proxy/. Some corporate proxies are
configured exactly this way.

Send auth whenever a username is present, defaulting password to
the empty string if url.Userinfo didn't set one.

Upstream PR #977.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: scalecode-solutions

proxy.go

@@ -58,11 +58,12 @@ func (hpd *httpProxyDialer) DialContext(ctx context.Context, network string, add
 
 	connectHeader := make(http.Header)
 	if user := hpd.proxyURL.User; user != nil {
+		// RFC 7617 permits an empty password. Send Basic auth whenever a
+		// username is present, even if no password was specified.
 		proxyUser := user.Username()
-		if proxyPassword, passwordSet := user.Password(); passwordSet {
-			credential := base64.StdEncoding.EncodeToString([]byte(proxyUser + ":" + proxyPassword))
-			connectHeader.Set("Proxy-Authorization", "Basic "+credential)
-		}
+		proxyPassword, _ := user.Password()
+		credential := base64.StdEncoding.EncodeToString([]byte(proxyUser + ":" + proxyPassword))
+		connectHeader.Set("Proxy-Authorization", "Basic "+credential)
 	}
 	connectReq := &http.Request{
 		Method: http.MethodConnect,