Fix #9359; set default WCS caching to Prefer Network

- This is noted in the tool tip, but was not set. Always Cache was the
default, which caused the cache to never be updated on upstream changes.

Caveat: this does not fix projects with existing WCS layers.

src/gui/qgsowssourceselect.cpp

@@ -86,6 +86,9 @@ QgsOWSSourceSelect::QgsOWSSourceSelect( QString service, QWidget * parent, Qt::W
   mCacheComboBox->addItem( tr( "Prefer network" ), QNetworkRequest::PreferNetwork );
   mCacheComboBox->addItem( tr( "Always network" ), QNetworkRequest::AlwaysNetwork );
 
+  // 'Prefer network' is the default noted in the combobox's tool tip
+  mCacheComboBox->setCurrentIndex( mCacheComboBox->findData( QNetworkRequest::PreferNetwork ) );
+
   if ( !mManagerMode )
   {
     connect( mAddButton, SIGNAL( clicked() ), this, SLOT( addClicked() ) );