code: function createWriteStream (path, options) {
      // build meta data
      var metadata = {}
      // get content type for file
      var contentType = mimeTypes.lookup(path)
      // set content type if found
      if (contentType) {
          metadata.contentType = contentType
      }
      // make sure bucket exists
      return this.bucketExists().then(() => {
          // create file write stream
          return this.bucket.file(path).createWriteStream({
              gzip: this.defaultGzip,
              metadata: metadata,
              public: this.defaultPublic,
              resumable: false,
          })
      })
  }
docs: @function createWriteStream
  
  @param {string} path
  @param {object} options
  
  @returns {Promise}