Fix issue when option isn't passed (#61)

Author: davidjamesstone

src/server/plugins/nunjucks/context.js

@@ -42,7 +42,7 @@ export function context(request) {
     throw Error('Missing baseLayoutPath in plugin.options.nunjucks')
   }
 
-  if ('viewContext' in pluginStorage) {
+  if (typeof pluginStorage.viewContext === 'function') {
     consumerViewContext = pluginStorage.viewContext(request)
   }