relayout: fix 'annotations[i].[key]': null bug

- previously all relayout calls with leading `annotations` or
  `shapes` and value `null` were replaced with value `'remove'`
- now make sure that only `annotations/shapes[i]: null` are
  replaced with `'remove'`

Author: etpinard

src/plot_api/plot_api.js

@@ -1917,13 +1917,14 @@ function _relayout(gd, aobj) {
                 objList = layout[objType] || [],
                 obji = objList[objNum] || {};
 
-            // new API, remove annotation / shape with `null`
-            if(vi === null) aobj[ai] = 'remove';
-
             // if p.parts is just an annotation number, and val is either
             // 'add' or an entire annotation to add, the undo is 'remove'
             // if val is 'remove' then undo is the whole annotation object
             if(p.parts.length === 2) {
+
+                // new API, remove annotation / shape with `null`
+                if(vi === null) aobj[ai] = 'remove';
+
                 if(aobj[ai] === 'add' || Lib.isPlainObject(aobj[ai])) {
                     undoit[ai] = 'remove';
                 }

test/jasmine/tests/annotations_test.js

@@ -86,4 +86,27 @@ describe('annotations relayout', function() {
             done();
         });
     });
+
+    it('should be able update annotations', function(done) {
+
+        function assertText(index, expected) {
+            var query = '.annotation[data-index="' + index + '"]',
+                actual = d3.select(query).select('text').text();
+
+            expect(actual).toEqual(expected);
+        }
+
+        assertText(0, 'left top');
+
+        Plotly.relayout(gd, 'annotations[0].text', 'hello').then(function() {
+            assertText(0, 'hello');
+
+            return Plotly.relayout(gd, 'annotations[0].text', null);
+        })
+        .then(function() {
+            assertText(0, 'new text');
+        })
+        .then(done);
+
+    });
 });