fix activeMarks for intersecting marks text (#2226)

packages/slate/src/models/text.js

@@ -307,7 +307,7 @@ class Text extends Record(DEFAULTS) {
     const result = this.leaves.first().marks
     if (result.size === 0) return result
 
-    return result.withMutations(x => {
+    return result.toOrderedSet().withMutations(x => {
       this.leaves.forEach(c => {
         x.intersect(c.marks)
         if (x.size === 0) return false

packages/slate/test/models/text/marks/get-active-marks/intersecting-marks-text.js

@@ -0,0 +1,25 @@
+/** @jsx h */
+
+import h from '../../../../helpers/h'
+import { Set } from 'immutable'
+import Mark from '../../../../../src/models/mark'
+
+export const input = (
+  <text>
+    <u>
+      <i>
+        <b>b</b>
+      </i>
+    </u>
+    <u>
+      <i>u</i>
+    </u>
+    <i>g</i>
+  </text>
+)[0]
+
+export default function(t) {
+  return t.getActiveMarks()
+}
+
+export const output = Set.of(Mark.create('italic'))