Flush in `File::write` when in unbuffered "text" mode

If the wrapped `QFile` was opened with `QIODevice::Unbuffered`, any
writes should be unbuffered.  However, as currently implemented,
using `QTextStream` when the `File` is in "text" mode causes all
reads/writes to be buffered.

This modification forces a flush in `File::write` if the wrapped
`QFile` was opened with `QIODevice::Unbuffered`.

Necessary to fix issue #11162 https://github.com/ariya/phantomjs/issues/11162.

src/filesystem.cpp

@@ -123,6 +123,9 @@ bool File::write(const QString &data)
     if ( m_fileStream ) {
         // text file
         (*m_fileStream) << data;
+        if (_isUnbuffered()) {
+            m_fileStream->flush();
+        }
         return true;
     } else {
         // binary file
@@ -213,6 +216,13 @@ void File::close()
     deleteLater();
 }
 
+// private:
+
+bool File::_isUnbuffered() const
+{
+    return m_file->openMode() & QIODevice::Unbuffered;
+}
+
 
 // FileSystem
 // public:

src/filesystem.h

@@ -65,6 +65,8 @@ public slots:
     void close();
 
 private:
+    bool _isUnbuffered() const;
+
     QFile *m_file;
     QTextStream *m_fileStream;
 };