BlueScreen exposes sensitive ENV variables

[dakujem]

Version: 2.6.x, 2.7.x

Bug Description

BlueScreen reveals sensitive server variables without offering option to turn this feature off or any kind of filtering option.
Furthermore, half of the class' code is private, so no easy customisation .... yet again.

Steps To Reproduce

Cause an exception to be rendered using BlueScreen, observe all server vars dumped at the bottom of the rendered screen.

Expected Behavior

Filtering of sensitive data enabled. Sensitive data not shown in plaintext.

Possible Solution

Refactor. Introduce options. Enable extensibility. Avoid private members and methods when no replacement is possible using composition.

Why Care

In cloud environments, it is common and/or necessary to store configuration options (like database connections, service keys, third party api tokens, etc. ) in environment variables. The described issue poses a security risk. Read more here: 12 factor app.

[dakujem]

This is quite possibly a security feature-issue. Why no response?

[dakujem]

I looked into this deeper and found out that the variables in the Environment section actually are filtered, except that no variable is matched (see the issue #431 for details). I prepared a PR #438 to patch this issue.