Update placeholder when it changes, not sure how this ever worked

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/components/views/rooms/BasicMessageComposer.js

@@ -94,6 +94,17 @@ export default class BasicMessageEditor extends React.Component {
         this._emoticonSettingHandle = null;
     }
 
+    componentDidUpdate(prevProps) {
+        if (this.props.placeholder !== prevProps.placeholder && this.props.placeholder) {
+            const {isEmpty} = this.props.model;
+            if (isEmpty) {
+                this._showPlaceholder();
+            } else {
+                this._hidePlaceholder();
+            }
+        }
+    }
+
     _replaceEmoticon = (caretPosition, inputType, diff) => {
         const {model} = this.props;
         const range = model.startRange(caretPosition);