Bug 817610 - (follow-up) Fast path for strings in Attributes
Bug 817610 - (follow-up) Fast path for strings in Attributes

Follow-up fix to bug 817610 to add the same optimization to Attributes and to
fix the regression it caused in Entity::_yield which should always return
a [locals, value] tuple.

projectfluent/fluent.js,projectfluent/fluent.js,Pike/l20n.js,projectfluent/fluent.js,Pike/l20n.js,mail-apps/l20n.js,zbraniecki/fluent.js,mail-apps/l20n.js,zbraniecki/l20n.js,stasm/l20n.js,l20n/l20n.js,zbraniecki/fluent.js,Swaven/l20n.js