Fix loading of external images (with canvas package installed)

Regressed in 37532c7 (v9.2.1). Fixes #1536.

Author: domenic

lib/jsdom/living/nodes/HTMLImageElement-impl.js

@@ -20,7 +20,7 @@ class HTMLImageElementImpl extends HTMLElementImpl {
           };
         }
         this._currentSrc = null;
-        resourceLoader.load(this, value, {}, (data, url, response) => {
+        resourceLoader.load(this, this.src, {}, (data, url, response) => {
           if (response && response.statusCode !== undefined && response.statusCode !== 200) {
             throw new Error("Status code: " + response.statusCode);
           }

test/living-html/htmlimageelement.js

@@ -1,6 +1,7 @@
 "use strict";
 const fs = require("fs");
 const path = require("path");
+const http = require("http");
 const jsdom = require("../..");
 const isCanvasInstalled = require("../util").isCanvasInstalled;
 const toFileUrl = require("../util").toFileUrl(__dirname);
@@ -46,6 +47,39 @@ exports["loading image from valid external URL"] = t => {
   };
 };
 
+exports["loading images should work with relative URLs (GH-1536)"] = t => {
+  if (!isCanvasInstalled(t)) {
+    return;
+  }
+
+  let requestsSoFar = 0;
+
+  const server = http.createServer((request, response) => {
+    if (requestsSoFar === 0) {
+      t.strictEqual(request.url, "/test.html");
+      response.end(`<img src="/test.jpg">`);
+    } else {
+      t.strictEqual(request.url, "/test.jpg");
+      response.end(``);
+      server.close();
+      t.done();
+    }
+
+    ++requestsSoFar;
+  })
+  .listen();
+
+  jsdom.env({
+    url: `http://127.0.0.1:${server.address().port}/test.html`,
+    features: {
+      FetchExternalResources: ["img"]
+    },
+    done(err) {
+      t.ifError(err);
+    }
+  });
+};
+
 exports["loading image from data URL"] = t => {
   if (!isCanvasInstalled(t)) {
     return;