rails-ujs: missing default url for ajax requests.

[DmytroVasin]

Steps to reproduce

Hi!
I'm using rails 5.1.0.rc1 and faced with next unexpected behaviour.

According to documentation I can set remote: true on select element like next:

<%= select_tag( "user_id", some_collection, ..., data: { remote: true, url: users_path } %>

And in this case everything will work.

But actually I can remove url

Expected behavior

When we use jquery_ujs and use $.ajax function:
This ajax function provides default value for url if it is not specified.

url (default: The current page)
Type: String
A string containing the URL to which the request is sent.

Actual behavior

Currently if I remove url option from select_tag I will receive an error:

rails-ujs.js:189 Uncaught TypeError: Cannot read property 'indexOf' of null
    at prepareOptions (rails-ujs.js:189)
    at Rails.ajax (rails-ujs.js:161)
    at HTMLSelectElement.Rails.handleRemote (rails-ujs.js:536)
    at HTMLDocument.<anonymous> (rails-ujs.js:137)
    at jQuery.fn.init.triggerNative (settings.js:96)
    at Object.<anonymous> (data-remote.js:435)
    at Test.run (qunit.js:1303)
    at qunit.js:1463
    at process (qunit.js:1016)
    at qunit.js:186

System configuration

Rails version:
5.1.0.rc1
Ruby version:
ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-darwin15]

Other Information

Maybe you had reasons not to do this.

Thanks.

[rails-bot]

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[maclover7]

@DmytroVasin Would you be able to add a test for this behavior?

[DmytroVasin]

Yes, i will

[rafaelfranca]

Could you squash your commits?

[DmytroVasin]

@rafaelfranca Done

[rafaelfranca]

Backported c7493fe