fix(g-image): run onload event once

Might be ralated to #93

gridsome/app/components/Image.js

@@ -143,17 +143,24 @@ function intersectionHandler ({ intersectionRatio, target }) {
 }
 
 function loadImage (el) {
+  const src = el.getAttribute('data-src')
+  const srcset = el.getAttribute('data-srcset')
+  const sizes = el.getAttribute('data-sizes')
+
+  if (!src) return
+
   el.onload = function () {
-    delete el.dataset.src
-    delete el.dataset.srcset
-    delete el.dataset.sizes
-    delete el.onload
+    el.removeAttribute('data-src')
+    el.removeAttribute('data-srcset')
+    el.removeAttribute('data-sizes')
 
     el.classList.remove('g-image--loading')
     el.classList.add('g-image--loaded')
+
+    delete el.onload
   }
 
-  el.src = el.dataset.src
-  el.srcset = el.dataset.srcset
-  el.sizes = el.dataset.sizes
+  el.src = src
+  el.srcset = srcset
+  el.sizes = sizes
 }