Fix assignments for scrub_headers and scrub_fields.

[jondeandres]

These assignments were done wrong, ||= should be used instead of |=, which is a binary operator. The response of both methods is probably not expected:

irb(main):001:0> a = nil
=> nil
irb(main):002:0> a |= 'this will not be set'
=> true
irb(main):003:0> a
=> true

This is mostly what could happen if Rollbar.configuration.scrub_fields is nil. Instead of assign a value to the config attribute I've used just || to return the first value if exists or the second.

[brianr]

This change looks good

[jondeandres]

@brianr changed