Focus form control elements during fill_in

Closes rubycdp#157
Related to rubycdp#280

Change the `lib/capybara/cuprite/javascripts/index.js` to invoke
[HTMLElement.focus][] directly, rather than triggering a synthetic
`"focus"` event.

[HTMLElement.focus]: https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/focus

Author: seanpdoyle

lib/capybara/cuprite/javascripts/index.js

@@ -153,7 +153,7 @@ class Cuprite {
 
     let valueBefore = node.value;
 
-    this.trigger(node, "focus");
+    node.focus();
     this.setValue(node, "");
 
     if (node.type == "number" || node.type == "date" || node.type == "range") {

spec/features/driver_spec.rb

@@ -1533,7 +1533,7 @@ def create_screenshot(file, *args)
         it "fills a date" do
           @session.fill_in "date_field", with: "2016-02-14"
 
-          expect(@session.find(:css, "#date_field").value).to eq("2016-02-14")
+          expect(@session.find(:css, "#date_field", focused: true).value).to eq("2016-02-14")
         end
       end