Don't set mustcapture for falsey values

Summary: See discussion in D3722. Good catch -- I'd definitely say this is a bug.

Test Plan: Rendered two <div />s, one with mustcapture set to `true` and one to `false`. The first got `data-mustcapture`; the second did not.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3723

src/infrastructure/javelin/markup.php

@@ -37,7 +37,11 @@ function javelin_render_tag(
           unset($attributes[$k]);
           break;
         case 'mustcapture':
-          $attributes['data-mustcapture'] = '1';
+          if ($v) {
+            $attributes['data-mustcapture'] = '1';
+          } else {
+            unset($attributes['data-mustcapture']);
+          }
           unset($attributes[$k]);
           break;
       }