Refactor filterSensitiveHeaders in lib/filters/http-headers.js

Re:

```
function filterSensitiveHeaders (headers) {
for (const key in headers) {
```

> Unrelated to this change: Wouldn't it typically be faster to iterate through the set of sensitive headers, rather than iterating through the set of given headers? I say "typically" because my guess is request and response headers generally number greater than 3. Anyway, if you agree, I can open a ticket to consider changing this. Low prio, this is possible premature optimization.