ProxyConnectionRefusedError is not available via QNetworkReply::error()

Steps to reproduce:

1. Call QNetworkAccessManager.setProxy() to configure HTTP proxy with some valid IP but non-listening port;
2. Create a QHttpRequest to some valid URL;
3. Call QNetworkAccessManager::get() with request from step 2;
4. On reply returned on step 3, connect to finished() signal;
5. When QNetworkReply::finished() is emitted, check the value return by error() and errorString()

Expected behavior:
QNetworkReply::error() == QNetworkReply::ProxyConnectionRefusedError
QNetworkReply::errorString() == "Proxy connection refused"

Actual behavior:
QNetworkReply::error() == QNetworkReply::UnknownNetworkError
QNetworkReply::errorString() == "Proxy connection refused"

Same behavior is observed with the QNetworkReply::error() signal parameter being UnknownNetworkError. Since this error code is non-specific, and error string is localizable and non-constant, there is no easy way to detect this (very common) proxy-related error.

Proposed fix:
In source file qhttpnetworkconnectionchannel.cpp, line 1049, add the following code:

case QAbstractSocket::ProxyConnectionRefusedError:
    errorCode = QNetworkReply::ProxyConnectionRefusedError;
    break;