Use aspect fill for image background

SwiftyStoreKit-iOS-Demo/Base.lproj/Main.storyboard

@@ -21,7 +21,7 @@
                         <rect key="frame" x="0.0" y="0.0" width="375" height="667"/>
                         <autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
                         <subviews>
-                            <imageView userInteractionEnabled="NO" contentMode="scaleAspectFit" horizontalHuggingPriority="251" verticalHuggingPriority="251" image="Background" translatesAutoresizingMaskIntoConstraints="NO" id="JDz-7n-4vD" userLabel="Background">
+                            <imageView userInteractionEnabled="NO" contentMode="scaleAspectFill" horizontalHuggingPriority="251" verticalHuggingPriority="251" image="Background" translatesAutoresizingMaskIntoConstraints="NO" id="JDz-7n-4vD" userLabel="Background">
                                 <rect key="frame" x="0.0" y="0.0" width="375" height="667"/>
                             </imageView>
                             <view contentMode="scaleToFill" translatesAutoresizingMaskIntoConstraints="NO" id="R2x-3P-rjx" userLabel="Opaque">