Attribute bind-value is not set in iron-input when initiated - data-binding

Question

Here's a pattern I try to do with Polymer:
<link rel="import" href="/bower_components/polymer/polymer.html" />
<link rel="import" href="/bower_components/iron-ajax/iron-ajax.html" />
<link rel="import" href="/bower_components/iron-input/iron-input.html" />
<dom-module id="my-entity">
<template>
<iron-ajax auto url="{{urlGet}}" handle-as="json" last-response="{{entity}}"></iron-ajax>
<iron-ajax id="updateEntityCall" url="{{urlUpdate}}" handle-as="json" on-response="handleUpdateResponse"></iron-ajax>
<div>
<div class="input-wrapper">
<label>Name : </label>
<input is="iron-input" bind-value="{{entityName}}" value="{{entity.name}}" />
</div>
<div>
<button on-click="saveEntity">Save</button>
</div>
</div>
</template>
<script>
Polymer(
{
is: "my-entity",
properties: {
entityid: {
notify: true
},
entityName: {
notify: true
}
urlGet: {
computed: 'getUrlGet(entityid)'
},
urlTemplate: {
computed: 'getUrlUpdate(entityid, entityName)'
},
},
getUrlGet: function (entityId) {
return ['/path/to/get', entityId].join('/');
},
getUrlUpdate: function (entityId, entityName) {
return ['/path/to/update', entityId, entityName].join('/');
},
saveEntity: function (e, detail, sender) {
this.$.updateEntityCall.generateRequest();
}
}
);
</script>
</dom-module>
I first create a iron-ajax that will get some data from an entity. This will be printed in a iron-input element so a person will be able to update this entity.
The iron-ajax is bound to a {{entity}} variable that is used by the value in the iron-input element (entity.name). I have created a property called entityName that will be bound at the bind-value attribute of this same iron-input.
Once someone has made the change, the goal is that he/she clicks on the button that calls the function saveEntity, that basically call the other iron-ajax element.
There is no problem with this code when someone actually change the value of the iron-input. The problem appear when the user don't change the value and click the button. It sends nothing, like if entityName is not set.
This is a simple example, in my use-case I have 3 fields and 2 of them can have "empty strings" as valid value.