Adds section header background view

Stops collapsing sections from appearing under the section view header.

iOS/MasterFeed/Cell/MasterFeedTableViewSectionHeader.swift

@@ -113,6 +113,7 @@ private extension MasterFeedTableViewSectionHeader {
 		addSubviewAtInit(titleView)
 		updateDisclosureImage()
 		addSubviewAtInit(disclosureView)
+		addBackgroundView()
 	}
 	
 	func updateDisclosureImage() {
@@ -134,4 +135,9 @@ private extension MasterFeedTableViewSectionHeader {
 		disclosureView.setFrameIfNotEqual(layout.disclosureButtonRect)
 	}
 	
+	func addBackgroundView() {
+		self.backgroundView = UIView(frame: self.bounds)
+		self.backgroundView?.backgroundColor = UIColor.systemGroupedBackground
+	}
+	
 }