change the property type of column className and cellClassName to be String only

[baljeet03]

This is regarding the below issue.
#416

That's my first PR in open source community :)

[alexander-alvarez]

Hi @baljeet03 I'm so sorry for not following up sooner.
I'm actually not sure about #416, so I'd need to review with @buschtoens
https://github.com/emberjs/ember.js/blob/v3.3.1/packages/ember-views/lib/mixins/class_names_support.js#L32 is an array, so as is should work.

[baljeet03]

@alexander-alvarez Though ember support it, we are adding this in classBindings and the passed array is being joined by comma , rather than space . So it will not work. One solution is to make this as computed property which will resolve this issue or we could just send as string only. I am open to both but will prefer computed one. I am open to suggestion :)

[alexander-alvarez]

Ooooh.. I see what you mean now, thanks for the clarification.

https://github.com/offirgolan/ember-light-table/blob/6ee059272b85b7efd2ff5acdead16ac00b8b4461/addon/components/columns/base.js#L23
https://github.com/offirgolan/ember-light-table/blob/v1.8.6/addon/components/cells/base.js#L25
https://github.com/offirgolan/ember-light-table/blob/6ee059272b85b7efd2ff5acdead16ac00b8b4461/addon/components/lt-row.js#L9

Like you mention, I think the computed property one would be best. Make sure to add tests so that we can make sure this works for the case as a string or as an array!