Assign class to link_to using html slim

I'm working on a html slim file for a Ruby on Rails project. I want to create a button of class btn btn-primary (I'm using bootstrap) for a controller action. Name of controller is default_responses and action is edit. So I first did:

= link_to 'Test this', :controller => 'default_responses', :action => 'edit', :id => params[:id].to_i

This would become

<a href="/default_responses/7/edit">Test this</a>

where 7 is the id parameter and is correct for my case. However, it is not a button at all, just an anchored tag. It also redirects me to the correct page.

Then I tried

 = link_to 'Test this', :controller => 'default_responses', :action => 'edit', :id => params[:id].to_i, class: 'btn btn-primary'

which gave me

<a href="/default_responses/7/edit?class=btn+btn-primary">Test this</a>

This is still not what I want as it is not a button still.

Also tried = link_to 'Test this', :controller => 'default_responses', :action => 'edit', :id => params[:id].to_i, :class=> 'btn btn-primary'

It returned <a href="/default_responses/7/edit?class=btn+btn-primary">Test this</a> which is still wrong.

Any suggestions? Thanks.