Sets the specified field in the hash stored at key to value. If the key does not exist, a new key holding a hash is created. If field already exists in the hash, it is overwritten.
|key holding the hash to set the field of.
|field to set in the hash.
|value to set the field to.
|On success, the promise resolves with the number of fields that were added.
|If the hash does not exist, the promise is rejected with an error.