Added empty content attribute to meta tags

Author: samcic

addon/templates/components/head-layout.hbs

@@ -1,3 +1,3 @@
 {{#-in-element headElement}}
-  <meta name="ember-cli-head-start">{{head-content}}<meta name="ember-cli-head-end">
+  <meta name="ember-cli-head-start" content="">{{head-content}}<meta name="ember-cli-head-end" content="">
 {{/-in-element}}