Use `+=` to be a bit more true to the spec language

spec/options/widget_lists_spec.rb

@@ -2,7 +2,7 @@ describe 'a zle widget' do
   let(:before_sourcing) { -> { session.run_command('my-widget() {}; zle -N my-widget; bindkey ^B my-widget') } }
 
   context 'when added to ZSH_AUTOSUGGEST_ACCEPT_WIDGETS' do
-    let(:options) { ['ZSH_AUTOSUGGEST_ACCEPT_WIDGETS=(my-widget)'] }
+    let(:options) { ['ZSH_AUTOSUGGEST_ACCEPT_WIDGETS+=(my-widget)'] }
 
     it 'accepts the suggestion when invoked' do
       with_history('echo hello') do
@@ -15,7 +15,7 @@ describe 'a zle widget' do
   end
 
   context 'when added to ZSH_AUTOSUGGEST_CLEAR_WIDGETS' do
-    let(:options) { ['ZSH_AUTOSUGGEST_CLEAR_WIDGETS=(my-widget)'] }
+    let(:options) { ['ZSH_AUTOSUGGEST_CLEAR_WIDGETS+=(my-widget)'] }
 
     it 'clears the suggestion when invoked' do
       with_history('echo hello') do
@@ -28,7 +28,7 @@ describe 'a zle widget' do
   end
 
   context 'when added to ZSH_AUTOSUGGEST_EXECUTE_WIDGETS' do
-    let(:options) { ['ZSH_AUTOSUGGEST_EXECUTE_WIDGETS=(my-widget)'] }
+    let(:options) { ['ZSH_AUTOSUGGEST_EXECUTE_WIDGETS+=(my-widget)'] }
 
     it 'executes the suggestion when invoked' do
       with_history('echo hello') do