Make OJP HTTP request content type customizable

VVO's TRIAS insists on a special value there.

src/lib/backends/openjourneyplannerbackend.cpp

@@ -132,7 +132,7 @@ bool OpenJourneyPlannerBackend::queryJourney(const JourneyRequest &request, Jour
 QNetworkRequest OpenJourneyPlannerBackend::networkRequest() const
 {
     QNetworkRequest req(m_endpoint);
-    req.setHeader(QNetworkRequest::ContentTypeHeader, QByteArray("application/xml"));
+    req.setHeader(QNetworkRequest::ContentTypeHeader, m_contentType);
     if (!m_authorization.isEmpty()) {
         req.setRawHeader("Authorization", m_authorization.toUtf8());
     }

src/lib/backends/openjourneyplannerbackend.h

@@ -9,6 +9,7 @@
 
 #include "abstractbackend.h"
 
+#include <QByteArray>
 #include <QUrl>
 
 class QNetworkRequest;
@@ -27,6 +28,8 @@ class OpenJourneyPlannerBackend : public AbstractBackend
     Q_PROPERTY(QString authorization MEMBER m_authorization)
     Q_PROPERTY(QString requestorRef MEMBER m_requestorRef)
     Q_PROPERTY(bool useTrias MEMBER m_useTrias)
+    /** Override the default HTTP ContentType header in the request. */
+    Q_PROPERTY(QByteArray contentType MEMBER m_contentType)
 
 public:
     static inline constexpr const char* type() { return "openJourneyPlanner"; }
@@ -44,6 +47,7 @@ private:
     QString m_authorization;
     QString m_requestorRef;
     bool m_useTrias = false;
+    QByteArray m_contentType = "application/xml";
 };
 
 }