Don't overwrite comment components if they're already set

Small fix. I'm finding that in a Rails 4.1 app (running rails-api) that the Marginalia instrumentation is being lazy loaded by ActiveSupport. This means it runs after I've set the comment components in my initializer, so they get overwritten. Switching the assignment to `||=` stops them being overwritten.

cc @arthurnn 
This seems unrelated to monkey patching the adapter.  Could we just move this to the Marginalia::Comment module?