Fix #1195: fix external images and fix test for it

lib/output/base.js

@@ -207,6 +207,10 @@ Output.prototype.onOutputSVG = function(page, svg) {
 // Output an image as a file
 // Normalize the relative link
 Output.prototype.onOutputImage = function(page, imgFile) {
+    if (location.isExternal(imgFile)) {
+        return imgFile;
+    }
+
     imgFile = page.resolveLocal(imgFile);
     return page.relative(imgFile);
 };

test/page.js

@@ -289,7 +289,7 @@ describe('Page', function() {
 
             it('should keep external images path', function() {
                 page.content.should.be.html({
-                    'img[src="https:/upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png"]': {
+                    'img[src="https://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png"]': {
                         count: 1
                     }
                 });