To reproduce:
1. Open attached project;
2. Enter "Test" scene;
3. Enter Play mode;
4. Copy <size=30>我</size><size=30>和</size><size=30>你</size> to the Input Field.
5. When Input Field is focused, press the Backspace key.

Expected result: when Input Field contains Rich Text with tags, backspace deletes the last letter, not the symbol of HTML tag.
Actual result: backspace deletes the last symbol of the HTML tag.

Reproduced on versions: 5.6.4p2, 5.6.4p4, 2017.1.3f1, 2017.2.1p1, 2017.3.0p1, 2018.1.0a7.