HTTP output "http_method" field is not interpolated.

Based on the general functionality of Logstash and the given documentation, it is expected the `http_method` field would be interpolated. However, attempting to do so results in an error like the following:

```
output {
http {
# This setting must be a ["put", "post", "patch", "delete", "get", "head"]
# Expected one of ["put", "post", "patch", "delete", "get", "head"], got ["%{[http][method]}"]
http_method => "%{[http][method]}"
...
}
}
```