Prefix custom _complete implementation with "function" keyword

For some reason, when `_complete` is aliased before sourcing the plugin,
zsh was blowing up with a parse error. Adding "function" keyword makes
it parse successfully.

See similar issue: https://github.com/robbyrussell/oh-my-zsh/issues/6723#issuecomment-381220834

Fixes GitHub #442

spec/strategies/completion_spec.rb

@@ -20,6 +20,23 @@ describe 'the `completion` suggestion strategy' do
     wait_for { session.content }.to eq("baz \\\nbar")
   end
 
+  context 'when `_complete` is aliased' do
+    let(:before_sourcing) do
+      -> do
+        session.
+          run_command('autoload compinit && compinit').
+          run_command('_foo() { compadd bar; compadd bat }').
+          run_command('compdef _foo baz').
+          run_command('alias _complete=_complete')
+      end
+    end
+
+    it 'suggests the first completion result' do
+      session.send_string('baz ')
+      wait_for { session.content }.to eq('baz bar')
+    end
+  end
+
   context 'when async mode is enabled' do
     let(:options) { ['ZSH_AUTOSUGGEST_USE_ASYNC=true', 'ZSH_AUTOSUGGEST_STRATEGY=completion'] }