Accessor trait fix: force string value.

System version 0.456
modules:
Blogs 0.084
Comments 0.024
Cron 0.008
Deferred tasks 0.1.2+build-5
Disqus 1.0.1+build-3
Feedback 0.006
HybridAuth 2.1.1-cs38
OAuth2 0.032
Photo_gallery 0.024
Plupload 2.1.1-cs1
Static_pages 0.042
plugins:
Admin_default_theme 0.005
Fotorama 4.4.9-cs1
SimpleImage 2.5.1-cs1
TinyMCE 4.0.16-cs1