Fix Spreadsheet editor when UI is displayed in Japanese

nuxeo-spreadsheet-button.html:131 Uncaught DOMException: Failed to execute 'btoa' on 'Window': The string to be encoded contains characters outside of the Latin1 range.
    at HTMLElement._show (https://dam-solution-tokyo.cloud.nuxeo.com/nuxeo//ui/nuxeo-spreadsheet/nuxeo-spreadsheet-button.html:131:90)
    at HTMLElement.handler (https://dam-solution-tokyo.cloud.nuxeo.com/nuxeo//ui/elements.html:2367:28)
    at Object._fire (https://dam-solution-tokyo.cloud.nuxeo.com/nuxeo//ui/elements.html:7373:14)
    at Object.forward (https://dam-solution-tokyo.cloud.nuxeo.com/nuxeo//ui/elements.html:7768:20)
    at Object.click (https://dam-solution-tokyo.cloud.nuxeo.com/nuxeo//ui/elements.html:7734:14)
    at HTMLElement._handleNative (https://dam-solution-tokyo.cloud.nuxeo.com/nuxeo//ui/elements.html:7157:19)

this.$.iframe.src = this.$.nxconn.url + '/spreadsheet/?cv=' + encodeURIComponent(btoa(JSON.stringify(state)));