Missing escape character of braces when building with Mathjax support

[haotian-wang]

The escape braces \{ and \} are converted to { and } when building with Mathjax support, which is incorrect.

Here is an example,

z = \max\{x, y\}

This should be rendered as

But mdBook renders it as

Some equations may even fail to render due to missing the escape character of the braces. For example,

z = \max\left\{x^2, y^2\right\}

This should be rendered as

mdBook will show it as