Fix a hyperscript bug in perserving text-key (#1846)

* Perserve the text key in the right way

* rename assertation to test

packages/slate-hyperscript/test/index.js

@@ -27,6 +27,7 @@ describe('slate-hyperscript', () => {
         const actual = input.toJSON()
         const expected = Value.isValue(output) ? output.toJSON() : output
         assert.deepEqual(actual, expected)
+        if (module.test) module.test()
       })
     }
   })