src/preferred-provider.js
function discoverFromHeaders (webId) {
  return fetch(webId, { method: 'OPTIONS' })
    .then(response => {
      if (response.ok) {
        return parseProviderLink(response.headers)
      }

      return null
    })
}
function discoverFromHeaders (webId) {
  return fetch(webId, { method: 'OPTIONS' })
    .then(response => {
      if (response.ok) {
        return parseProviderLink(response.headers)
      }

      return null
    })
}
@param webId {string}

@returns {Promise<string|null>}