Fix issue where "{Fnnn, size}" renders as though "size=full"

Summary: We interpret "size" as "size => true", and "true == 'full'", so we hit the wrong branch in the switch(). String cast explicitly.

Test Plan: Typed `{Fnnn, size}`; saw it render as a thumb instead of full.

Reviewers: btrahan, codeblock

Reviewed By: codeblock

CC: frozendevil, aran

Differential Revision: https://secure.phabricator.com/D4323

src/infrastructure/markup/rule/PhabricatorRemarkupRuleEmbedFile.php

@@ -50,7 +50,7 @@ final class PhabricatorRemarkupRuleEmbedFile
     $options['name'] = $file_name;
 
     $attrs = array();
-    switch ($options['size']) {
+    switch ((string)$options['size']) {
       case 'full':
         $attrs['src'] = $file->getBestURI();
         $options['image_class'] = null;