Fix args passed to image_tag in ViewHelpers spec

Deprecated negative raise_error expectations were removed by Transpec
in previous commit 26ade602: this example was previously masking an
ArgumentError.

Signed-off-by: Alex Coles <alex@alexbcoles.com>

spec/lib/open_project/themes/view_helpers_spec.rb

@@ -35,7 +35,7 @@ module OpenProject
 
       it "is mixed into application controller's helper chain" do
         expect { helpers.current_theme }.to_not raise_error
-        expect { helpers.image_tag }.to_not raise_error
+        expect { helpers.image_tag('example.png') }.to_not raise_error
       end
 
       describe '#current_theme' do