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;
@@ -173,6 +176,10 @@ 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', $_SERVER['HTTP_IF_UNMODIFIED_SINCE']);
+    if ($unmodified_since === FALSE) {
+      return $return;
+    }
+    else {
       if ($datastream->createdDate->getTimestamp() !== $unmodified_since->getTimestamp()) {
         // Changed!
         $return = 412;
@@ -181,6 +188,7 @@ function islandora_view_datastream_cache_check(AbstractDatastream $datastream) {
         return $return;
       }
     }
+  }
 
   // Only consider Etags we have provided.
   if (isset($datastream->checksum)) {