fix(selection): Revert to jQuery-behavior for link attrs update

Revert to same behavior as jQuery's .attr() for updating attributes in link():
- null value removes attribute
- undefined value does not modify attribute

This reverts to the behavior before the migration away from jQuery (83e512f).

Author: romankaravia

spec/selection.spec.js

@@ -269,6 +269,27 @@ describe('Selection', function () {
         const linkTags = this.selection.getTagsByName('a')
         expect(linkTags.length).to.equal(0)
       })
+
+      it('sets class attribute', function () {
+        this.selection.link('https://livingdocs.io', {class: 'baz'})
+        const linkTags = this.selection.getTagsByName('a')
+        const html = getHtml(linkTags[0])
+        expect(html).to.equal('<a class="baz" href="https://livingdocs.io">foobar</a>')
+      })
+
+      it('removes class attribute when set to null', function () {
+        this.selection.link('https://livingdocs.io', {class: null})
+        const linkTags = this.selection.getTagsByName('a')
+        const html = getHtml(linkTags[0])
+        expect(html).to.equal('<a href="https://livingdocs.io">foobar</a>')
+      })
+
+      it('does not modify class attribute when set to undefined', function () {
+        this.selection.link('https://livingdocs.io', {class: undefined})
+        const linkTags = this.selection.getTagsByName('a')
+        const html = getHtml(linkTags[0])
+        expect(html).to.equal('<a class="foo bar" href="https://livingdocs.io">foobar</a>')
+      })
     })
 
   })

src/selection.js

@@ -78,7 +78,15 @@ export default class Selection extends Cursor {
   link (href, attrs = {}) {
     if (href) attrs.href = href
     const link = this.createElement(config.linkMarkup.name, config.linkMarkup.attribs)
-    for (const key in attrs) link.setAttribute(key, attrs[key])
+    for (const key in attrs) {
+      const value = attrs[key]
+      if (value === undefined) continue
+      if (value === null) {
+        link.removeAttribute(key)
+      } else {
+        link.setAttribute(key, value)
+      }
+    }
     this.forceWrap(link)
   }