Truncate Mutates Context Data about apm-nodejs-http-client

[LucGranato]

This gave me a brain aneurysm.
Lucky that APM was the only thing that was different between the local test and the production environment. It was easier to guess...
Going back to version 2.5.1 fixed the problem.

[blakeembrey]

@watson Is there any context here for fixing this? I've gone through and fixed it up on my side, but I can't open a working PR because breadth-filter means either making a trade-off of cloning context on a new object and failing the circular reference tests, or stringifying within the breadth-filter callback but it automatically traverses the headers object test case and fails because we don't stringify that. Are either of these trade-offs acceptable? Alternatively, I can update breadth-filter to always call fn on every value (even object and array), which would solve for both these issues.

Edit: Also, alternatively, could just add validation onto context.{request,response}.headers instead to ensure they're the shape expected. Why would anyone use this with unexpected input?