fix: support non-canonicalised srcset attribute name #2958 (#2968)

Some frameworks, like React, use non-canonicalised attribute names. The
spec and browsers allow this, but hammerhead is only checking if the
attribute name without canonicalisation matches `srcset`.

By using the `loweredAttr` for the comparison, hammerhead will be more
accepting of inputs and resolve srcset issues with React projects
(fixes #2958).

See facebook/react#10863 for more background.

src/client/sandbox/node/element.ts

@@ -152,7 +152,7 @@ export default class ElementSandbox extends SandboxBase {
         const tagName     = domUtils.getTagName(el);
         const isUrlAttr   = domProcessor.isUrlAttr(el, attr, ns);
         const isEventAttr = domProcessor.EVENTS.indexOf(attr) !== -1;
-        const isUrlsSet   = attr === 'srcset';
+        const isUrlsSet   = loweredAttr === 'srcset';
 
         let needToCallTargetChanged = false;
         let needToRecalcHref        = false;

test/client/fixtures/sandbox/node/methods-test.js

@@ -243,6 +243,16 @@ test('setAttribute: img src', function () {
     strictEqual(nativeMethods.getAttribute.call(img, 'src'), urlUtils.resolveUrlAsDest('/image.gif?param=value'));
 });
 
+test('setAttribute: img srcSet', function () {
+    var img = nativeMethods.createElement.call(document, 'img');
+
+    processDomMeth(img);
+
+    img.setAttribute('srcSet', '/image.gif?param=value 1x, /imagex2.gif?param=value 2x');
+
+    strictEqual(nativeMethods.getAttribute.call(img, 'srcset'), urlUtils.resolveUrlAsDest('/image.gif?param=value 1x, /imagex2.gif?param=value 2x', true));
+});
+
 test('canvasRenderingContext2D.drawImage', function () {
     var storedNativeMethod  = nativeMethods.canvasContextDrawImage;
     var crossDomainUrl      = 'http://crossdomain.com/image.png';