Remove duplicates on methods discovered by allowed_methods.py plugin

Example allowed_methods.py output before this change:
The URL "http://w3/" has the following enabled HTTP methods: CONNECT, GET, GET,
HEAD, HEAD, OPTIONS, OPTIONS, POST, POST. This information was found in the
requests with ids 32, 39, 47, 52, 55 and 71.

Example allowed_methods.py output after this change:
The URL "http://w3/" has the following enabled HTTP methods: CONNECT, GET, HEAD,
OPTIONS, POST. This information was found in the requests with ids 32, 39, 47,
52, 55 and 71.

Note that duplicate methods are removed and request ID list is kept unchanged.
We want to keep all IDs to see related requests no matter what the discovery
method was (bruteforce or an OPTIONS request)