fix hitting return key in HeaderView

This fixes return key behaviour in Chrome. Steps to reproduce:
- open Swagger UI in Chrome
- modify URL of swagger spec
- hit 'return'

Expected behaviour: load swagger.json from new URL
Observed behaviour before applying this fix: the whole page will reload
with the default swagger petstore URL in the input field.

dist/swagger-ui.js

@@ -22223,6 +22223,7 @@ SwaggerUi.Views.HeaderView = Backbone.View.extend({
   events: {
     'click #show-pet-store-icon'    : 'showPetStore',
     'click #explore'                : 'showCustom',
+    'submit #api_selector'          : 'showCustom',
     'keyup #input_baseUrl'          : 'showCustomOnKeyup',
     'keyup #input_apiKey'           : 'showCustomOnKeyup'
   },

src/main/javascript/view/HeaderView.js

@@ -4,6 +4,7 @@ SwaggerUi.Views.HeaderView = Backbone.View.extend({
   events: {
     'click #show-pet-store-icon'    : 'showPetStore',
     'click #explore'                : 'showCustom',
+    'submit #api_selector'          : 'showCustom',
     'keyup #input_baseUrl'          : 'showCustomOnKeyup',
     'keyup #input_apiKey'           : 'showCustomOnKeyup'
   },