Cannot correctly handle a case with pathological braces nesting

Consider the following case

``` wiki
{{{{{arg}}
```

which should be parsed as

```
[{{{] {{ [arg] }}
```

But my parser handles it as

```
[{] {{{ [{arg] }}
```

The corresponding test case is [UnitTestProject1.AbnormalCaseTests.TestBraces3](https://github.com/CXuesong/MwParserFromScratch/blob/master/UnitTestProject1/AbnormalCaseTests.cs#L151).
