Unable to Databind localstorage to iron-ajax headers - polymer

Question

I'm using the latest Polymer (1.2.0), and I'm having trouble with databinding from iron-localstorage to the iron-ajax headers field. I'm not seeing the Authorization header set when I inspect the request. I've verified the request works when I just create a valid headers object with no databinding.
Am I doing something wrong, or is it not designed to be used like this?
<iron-localstorage name="userToken" value="{{localtoken}}" use-raw></iron-localstorage>
<iron-ajax url="api/twitter/v1/private/gettweets" last-response="{{data}}" auto
headers= '{"Authorization":"Bearer [[localtoken]]}"'
handle-as="json">
</iron-ajax>
<iron-list items="[[data.futuretweets]]" as="item">
<template>
<div>
datetime: <span>[[item.datetime]]</span>
text: <span>[[item.text]]</span>
</div>
</template>
</iron-list>

Answer 1

I think you have a typo error in your compound binding, here is a corrected version:
<iron-ajax url="api/twitter/v1/private/gettweets" last-response="{{data}}" auto
headers= '{"Authorization":"Bearer [[localtoken]]"}'
handle-as="json">
</iron-ajax>
[EDIT] since it is not working, try with a computed function like this:
<iron-ajax url="api/twitter/v1/private/gettweets" last-response="{{data}}" auto
headers='_computeHeaders(localtoken)'
handle-as="json">
</iron-ajax>
where
_computeHeaders(localtoken) {
return {"Authorization": "Bearer " + localtoken};
}

Answer 2

Shouldn't:
_computeHeaders(localtoken) {
return '{"Authorization":"Bearer ' + localtoken + '"}';
}
Instead be:
_computeHeaders: function(localtoken){
return '{"Authorization":"Bearer ' + localtoken + '"}';
}

Answer 3

I've found a little workaround.
I'm using loopback with authentication service enabled and I'm saving the authentication token with iron-localstorage. The problem I've found is that the "auto" of iron-ajax let start the request when any ( almost ) of the iron-ajax parameter change. When the request start the localstorage value hasn't been populated yet
<iron-localstorage name="appdata" value="{{user}}"></iron-localstorage>
and
<iron-ajax auto="{{user.id}}" headers$='{"Authorization" :"{{user.id}}"}' url="/api/data" handle-as="json" last-response="{{data}}"></iron-ajax>
the workaround is inside auto="{{user.id}}". While user isn't loaded the user.id is false. When loaded it become something that match as true. That cause also a change inside the iron-ajax header attribute and cause the "auto" request send to get fired.