Feature/220

Adding `HEAD`, `OPTIONS`, `TRACE` to the list of uncacheable request methods.

- Method `is_post_put_del_request()` renamed to `is_uncacheable_request_method()`.
- Renaming `NC_DEBUG_POST_PUT_DEL_REQUEST` TO `NC_DEBUG_UNCACHEABLE_REQUEST` and updating it's detailed description to include mention of `HEAD`, `OPTIONS` and `TRACE`.