Space-separated classes with btn-primary do not keep btn-default from being added

[marcp]

button_to('Test', '/test', class: 'xyz btn-primary') adds 'btn-default' to the generated html.

button_to('Test', '/test', class: ['xyz', 'btn-primary']) does not add 'btn-default' to the generated html.

I have a patch that makes the first one do the same thing the second one does.

[doabit]

@marcp Welcome pr.