Check for custom body response

[fabge]

Currently there exists following method to check for specific error messages in the body:

sites:
  - name: Custom down
    url: https://example.com
    __dangerous__body_down: "File not found"

We use a software which returns following body when running a query against it's health API:

{'health': 'GREEN', 'causes': []}

The API might also return a 200 response code when there is an error and instead return a different body.
So it would be useful to check for the opposite - e.g. string that should exist in the body.

[fabge]

I just saw, that this has already been answered in 157.
It's just missing from the docs.

[AnandChowdhary]

Can you please make a PR to add it to the docs? Much appreciated! :)

[fabge]

@AnandChowdhary been there done that
upptime/upptime.js.org#33