reverse selection gradient when opened up to have better separation. fixes select2#1040

Author: ivaynberg

select2.css

@@ -301,6 +301,20 @@ Version: @@ver@@ Timestamp: @@timestamp@@
     background-image: linear-gradient(top, #ffffff 0%,#eeeeee 50%);
 }
 
+.select2-dropdown-open.select2-drop-above .select2-choice,
+.select2-dropdown-open.select2-drop-above .select2-choices {
+    border: 1px solid #5897fb;
+    border-top-color: transparent;
+
+    background-image: -webkit-gradient(linear, left top, left bottom, color-stop(0, white), color-stop(0.5, #eeeeee));
+    background-image: -webkit-linear-gradient(center top, white 0%, #eeeeee 50%);
+    background-image: -moz-linear-gradient(center top, white 0%, #eeeeee 50%);
+    background-image: -o-linear-gradient(top, white 0%, #eeeeee 50%);
+    background-image: -ms-linear-gradient(bottom, #ffffff 0%,#eeeeee 50%);
+    filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#eeeeee', endColorstr='#ffffff',GradientType=0 );
+    background-image: linear-gradient(bottom, #ffffff 0%,#eeeeee 50%);
+}
+
 .select2-dropdown-open .select2-choice div {
     background: transparent;
     border-left: none;