This fixes a bug that, when a span covers several nodes and a long node
has already been candidate-fixed, fixing a short node does not cause
the walk to reflect the result.
A concrete example:
1. type 高中生.
2. move the cursor to 中 and change to 鐘聲: 高鐘聲.
3. with cursor position unchanged, select the candidate to 忠.
4. the expected result should be 高忠生 but instead it is stuck with
高鐘聲 due to the node representing "鐘聲" is still fixed.
Fixes#54
Lukhnos Liu
Mengjuei