quote alt text of block image

lib/document-model-converter.js

@@ -131,7 +131,7 @@ class DocumentModelConverter {
       if (target.endsWith('.svg')) {
         return this.baseConverter.convert(node, transform)
       }
-      const img = `<img src="${node.getImageUri(target)}" alt=${encodeAttributeValue(node.getAlt())}/>`
+      const img = `<img src="${node.getImageUri(target)}" alt="${encodeAttributeValue(node.getAlt())}"/>`
       const mimeType = target.endsWith('png') ? 'image/png' : 'image/jpeg'
       const targetInfo = parse(target)
       return `<div class="imageblock">