Fix RuboCop minor issues

Author: blocknotes

.rubocop.yml

@@ -34,3 +34,12 @@ RSpec/ExampleLength:
 RSpec/MultipleMemoizedHelpers:
   # default: 5
   Max: 6
+
+RSpec/SubjectStub:
+  Exclude:
+    - spec/units/prawn_html/attributes_spec.rb
+
+Style/OpenStructUse:
+  Exclude:
+    - lib/prawn_html/attributes.rb
+    - spec/units/prawn_html/attributes_spec.rb

spec/units/prawn_html/utils_spec.rb

@@ -127,13 +127,13 @@
     context 'with a nil value' do
       let(:value) { nil }
 
-      it { is_expected.to eq nil }
+      it { is_expected.to be_nil }
     end
 
     context 'with a blank string value' do
       let(:value) { '' }
 
-      it { is_expected.to eq nil }
+      it { is_expected.to be_nil }
     end
 
     context 'with a string value (ex. "some_string")' do
@@ -161,7 +161,7 @@
     context 'with an invalid value (ex. "some_string")' do
       let(:value) { 'some_string' }
 
-      it { is_expected.to eq nil }
+      it { is_expected.to be_nil }
     end
 
     context 'with a valid value (ex. " bOlD ")' do