Use keyword arguments in mappable specs

We were getting a warning with Ruby 2.7 due to the change in the way
keyword arguments are handled in Ruby 3.0.

```
ruby/gems/2.7.0/gems/rspec-support-3.11.0/lib/rspec/support/with_keywords_when_needed.rb:18:
warning: Passing the keyword argument as the last hash parameter is
deprecated
```

As hinted by the warning, this code crashes with Ruby 3.0:

```
ArgumentError:
  unknown keyword: :budget_id
```

I'm not sure why this is the case, though, since we were already
explicitely passing a hash first before passing the keyword parameters.
I guess there are some cases in this whole keyword VS hash parameters
incompatibility that I haven't completely understood.

spec/system/proposals_spec.rb

@@ -1296,8 +1296,7 @@ describe "Proposals" do
                   "proposal",
                   "new_proposal_path",
                   "edit_proposal_path",
-                  "proposal_path",
-                  {}
+                  "proposal_path"
 
   scenario "Erased author" do
     user = create(:user)