JSON.stringify() container cradle results in memory leak because of cyclic references

[berndartmueller]

Winston, sentry, and many more libraries, which call internally JSON.stringify or some other (custom) function for JSON serialization, cause a memory leak when passed a container cradle.

I especially had the case together with Sequelize and winston json formatter. For some reason, sometimes (mostly in case of errors) it tried to log the whole container cradle. This took forever and resulted in a silent app crash which was not immediately traced back to the problems roots.

This PR simply checks if the container cradle was called via JSON.stringify by checking for toJSON and returns a simple string.

Following github issues are connected to this:
coopernurse/node-pool#275
sequelize/sequelize#10443

Thanks!

[coveralls]

Coverage remained the same at 100.0% when pulling 7849bf4 on findheim:feat-cradle-toJSON into ab5c033 on jeffijoe:master.

[jeffijoe]

Thanks! Released as v4.2.3! 🎉