nuxeo-operation-button problem with "download" option

A nuxeo-operation-button that invokes a void operation will display "TypeError: response.headers.get is not a function" in the Notification message regardless of whether the "download" property is set to "true" or "false". If the "download" property is not used this does not happen.

Since the "download" property is a boolean, Studio must generate <nuxeo-operation-button download> if the download checkbox is checked and <nuxeo-operation-button > (no download property) if the checkbox is not checked.

In the scope of this work it would be smart to find other incidents where a boolean property is used and make sure Studio generates the right code for them.