Keep sort preference for previously sorted columns

[wanam]

This fixes #219

[BeyondEvil]

Thanks @wanam

Could you add a test for this as well?

[wanam]

I added 2 assertions to check this issue on sort_column test.

[BeyondEvil]

Could you update with master and fix failing tests @wanam ?

[wanam]

@BeyondEvil I'm getting some troubles trying to install your CI environment because of a grunt-contrib-qunit's dependency 'puppeteer' that fails to download Chromium module, maybe because I'm behind a proxy...

Anyway executing the tests on Chrome browser gave me the same result with the 3 failed tests, this cannot reproduced on my default browser: Firefox, do you have any idea what could be the reason for this?

[BeyondEvil]

js tests and related issues are not my fortay. Maybe @RibeiroAna has some input?

If not, I'll see if I can illicit some help from a colleague. 🤷‍♂

Probably what we should do is replace phantomjs. 🤔

[wanam]

I still don't understand why we have different behavior on Firefox vs Chrome, but I just pushed a fix for the sorting issue, the root cause was in case of equal keys, for this case the sort function should return 0.

@BeyondEvil If it's ok to merge, could you squash my commits into one commit?

[BeyondEvil]

LGTM!