fix payload strip of invalid utf8 sequences from symbols besides strings

This is a fix for invalid utf8 sequences in payload symbols according to #85. Please note we cannot modify the value in-place as there are frozen strings and also we need to convert back to symbol, otherwise half of the specs are failing.
Hey, thanks for these changes. Actually it's a simpler method and easier to read, IMHO. Just one thing, the tests are failing cause in the code we are doing `return object unless object == object.to_s || is_symbol` and we are using `proc` instead of `lambda`.

I've checked the change here, https://travis-ci.org/jondeandres/rollbar-gem/builds/36208349, so you just need to replace `proc` with `lambda`.