[fix] encode resourceURL

[igtm]

What is the problem?

a Fatal error occurs when a resource url has some unescaped characters which must be escaped before unwrapping.
For example, https://fonts.googleapis.com/css?family=Google+Sans:400,500|Roboto:300,400,500|Roboto+Mono:400,700|Material+Icons.

I encounted the error when I tried to run the InAppBrowser example code.

What is my solution?

encoding urls before unwrapping.

my environment

Flutter 1.1.2-pre.39 • channel master • https://github.com/flutter/flutter.git
Framework • revision 0e9ad43416 (34 hours ago) • 2018-12-16 16:39:28 -0500
Engine • revision 4941125829
Tools • Dart 2.2.0 (build 2.2.0-dev.1.1 f9ebf21297)

[pichillilorenzo]

Thanks! I will merge it as soon as possible! Before I need to review another pull request and then I will merge this one 👍