minitrue is a proxy designed to lie.
It does this by mangling the requests sent by the client before forwarding them to the remote server, and mangling the responses sent by the server before relaying them back to the client.
Note that this is little more than a proof of concept. If you want to do this to any amount of traffic, there are better alternatives, such as icap in combination with a proxy server such as Squid.
Like so many other tools, there are evil use cases for this tool. What you use it for is entirely up to you.
The code is available under the very permissive ISC license. Contribution is welcomed through the Github project page.
This project doesn't come with a lot of documentation yet. Maybe, one day, some documentation will be written. If you want to use it right now, the test suite is a good place to start.