Code review feedback

Return the current value of $return if we have a bad value.

includes/datastream.inc

@@ -161,7 +161,10 @@ function islandora_view_datastream_cache_check(AbstractDatastream $datastream) {
 
   if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
     $modified_since = DateTime::createFromFormat('D, d M Y H:i:s e', $_SERVER['HTTP_IF_MODIFIED_SINCE']);
-    if ($modified_since !== FALSE) {
+    if ($modified_since === FALSE) {
+      return $return;
+    }
+    else {
       if ($datastream->createdDate->getTimestamp() - $modified_since->getTimestamp() > 0) {
         // Changed!
         return $return;
@@ -173,12 +176,17 @@ function islandora_view_datastream_cache_check(AbstractDatastream $datastream) {
   }
   if ($return === 200 && isset($_SERVER['HTTP_IF_UNMODIFIED_SINCE'])) {
     $unmodified_since = DateTime::createFromFormat('D, d M Y H:i:s e', $_SERVER['HTTP_IF_UNMODIFIED_SINCE']);
-    if ($datastream->createdDate->getTimestamp() !== $unmodified_since->getTimestamp()) {
-      // Changed!
-      $return = 412;
+    if ($unmodified_since === FALSE) {
+      return $return;
     }
     else {
-      return $return;
+      if ($datastream->createdDate->getTimestamp() !== $unmodified_since->getTimestamp()) {
+        // Changed!
+        $return = 412;
+      }
+      else {
+        return $return;
+      }
     }
   }