Fix assignments for scrub_headers and scrub_fields.

These assignments were done wrong, `||=` should be used instead of `|=`, which is a binary operator. The response of both methods is probably not expected:

```ruby
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.
I'm not sure if it was originally done on purpose, but the old version does have the behavior that `Rollbar.configuration.scrub_fields` would be combined with the parameter_filter:

```
irb(main):006:0> a = ['hello']
=> ["hello"]
irb(main):007:0> a |= ['world']
=> ["hello", "world"]
irb(main):008:0> a |= ['world']
=> ["hello", "world"]
```