fix(ViewController): avoid deep copy in dismiss() that can turn into an infinite loop

For some reason, apparently related to the NavOptions.ev field,
this deep copy was turning into an infinite loop on a Nexus 7 / android 5.1.1 webview.
Replace with a non-deep copy since the deep copy is unnecessary in this case.

Author: vanackere

src/navigation/view-controller.ts

@@ -1,7 +1,7 @@
 import { ComponentRef, ElementRef, EventEmitter, Output, Renderer } from '@angular/core';
 
 import { Footer, Header } from '../components/toolbar/toolbar';
-import { isPresent, merge } from '../util/util';
+import { isPresent, assign } from '../util/util';
 import { Navbar } from '../components/navbar/navbar';
 import { NavControllerBase } from './nav-controller-base';
 import { NavOptions, ViewState } from './nav-util';
@@ -184,7 +184,7 @@ export class ViewController {
       return Promise.resolve(false);
     }
 
-    let options = merge({}, this._leavingOpts, navOptions);
+    let options = assign({}, this._leavingOpts, navOptions);
     this._onWillDismiss && this._onWillDismiss(data, role);
     return this._nav.remove(this._nav.indexOf(this), 1, options).then(() => {
       this._onDidDismiss && this._onDidDismiss(data, role);