fix: URL-decode proxy username and password

In cases where the proxy username or password contains characters that
cannot show up in URLs, they should be URL-encoded.

This is to ensure backwards compatibility with the Mender client
3.

Unfortunately, tinyproxy considers all special characters in the
BasicAuth configuration entry as syntax error so we have no way
to test this.

Ticket: MEN-7402
Changelog: none
Signed-off-by: Vratislav Podzimek <vratislav.podzimek@northern.tech>

src/common/http/platform/beast/http.cpp

@@ -444,7 +444,7 @@ static inline error::Error AddProxyAuthHeader(OutgoingRequest &req, BrokenDownUr
 		// nothing to do
 		return error::NoError;
 	}
-	auto creds = proxy_address.username + ":" + proxy_address.password;
+	auto creds = URLDecode(proxy_address.username) + ":" + URLDecode(proxy_address.password);
 	auto ex_encoded_creds = crypto::EncodeBase64(common::ByteVectorFromString(creds));
 	if (!ex_encoded_creds) {
 		return ex_encoded_creds.error();