Circular references processor should not change user data

[estepnv]

I understand that it's not memory-efficient, but you are working with business rules which might rely on references.
Consider opting out this behavior, or display a warning somewhere, and this behavior should be documented.
I spent 6 hours of debugging code because of the sentry event. Imagine my feelings when I figure out it's caused by raven.
Fixes https://github.com/getsentry/raven-ruby/issues/903

[st0012]

@estepnv thanks for the fix!
but this approach could increase memory allocation by 16% or more, which is something we should avoid if possible (you can see the detail about this analysis at https://github.com/getsentry/raven-ruby/pull/994's description). so I will close this once #994 is proved working and merged.