Put in a general fix for #219 - Handling input from zle -U

Depends on patch to ZSH from workers/40702:

  http://www.zsh.org/mla/workers/2017/msg00414.html

spec/integrations/zle_input_stack_spec.rb

@@ -0,0 +1,24 @@
+describe 'using `zle -U`' do
+  let(:before_sourcing) do
+    -> do
+      session.
+        run_command('_zsh_autosuggest_strategy_test() { sleep 1; _zsh_autosuggest_strategy_default "$1" }').
+        run_command('foo() { zle -U - "echo hello" }; zle -N foo; bindkey ^B foo')
+    end
+  end
+
+  let(:options) { ['unset ZSH_AUTOSUGGEST_USE_ASYNC', 'ZSH_AUTOSUGGEST_STRATEGY=test'] }
+
+  # TODO: This is only possible with the $KEYS_QUEUED_COUNT widget parameter, coming soon...
+  xit 'does not fetch a suggestion for every inserted character' do
+    session.send_keys('C-b')
+    wait_for { session.content }.to eq('echo hello')
+  end
+
+  it 'shows a suggestion when the widget completes' do
+    with_history('echo hello world') do
+      session.send_keys('C-b')
+      wait_for { session.content(esc_seqs: true) }.to match(/\Aecho hello\e\[[0-9]+m world/)
+    end
+  end
+end