Martian is a library for building custom HTTP/S proxies

 Martian Proxy is a programmable HTTP proxy designed to be used for testing.
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 Martian is a great tool to use if you want to:
  * Verify that all (or some subset) of requests are secure
  * Mock external services at the network layer
  * Inject headers, modify cookies or perform other mutations of HTTP requests
    and responses
  * Verify that pingbacks happen when you think they should
  * Unwrap encrypted traffic (requires install of CA certificate in browser)