fix(polyfill): do not consider URLs viable values for running the polyfill. (#78)

I actually got a bug with background-image property
pointing to cloudfront with this url:
  https://d3r0d0kb2vhuxd.cloudfront.net

The viewportUnitExpression regex matched the url
and the url got rewritten.

This is a fix to avoid that.

Author: jfabre

viewport-units-buggyfill.js

@@ -27,6 +27,7 @@
     var options;
     var userAgent = window.navigator.userAgent;
     var viewportUnitExpression = /([+-]?[0-9.]+)(vh|vw|vmin|vmax)/g;
+    var urlExpression = /(https?:)?\/\//
     var forEach = [].forEach;
     var dimensions;
     var declarations;
@@ -261,7 +262,7 @@
         }
 
         viewportUnitExpression.lastIndex = 0;
-        if (viewportUnitExpression.test(value)) {
+        if (viewportUnitExpression.test(value) && !urlExpression.test(value)) {
           // KeyframesRule does not have a CSS-PropertyName
           declarations.push([rule, null, value]);
           options.hacks && options.hacks.findDeclarations(declarations, rule, null, value);