Fix proxying header assertion failure.

Re-write Server of proxied requests to avoid:
    coolwsd: ./net/HttpRequest.hpp:945: bool http::Response::writeData(Buffer &) const:
    Assertion `get("Server") == http::getServerString() &&
    "Server Agent is always set in http::Response ctor"' failed.

Change-Id: I6076dbc355b0f42a6367271893c9732195c8eb2a
Signed-off-by: Michael Meeks <michael.meeks@collabora.com>

wsd/ProxyRequestHandler.cpp

@@ -65,6 +65,9 @@ void ProxyRequestHandler::handleRequest(const std::string& relPath,
 
                         CacheFileHash[httpSession->getUrl()] = httpResponse;
 
+                        // We're proxying, we take responsibility.
+                        httpResponse->set("Server", http::getServerString());
+
                         socket->sendAndShutdown(*httpResponse);
                     }
                     else