From the HTTP 1.0 specification on the Request-URI:
The absoluteURI form is only allowed when the request is being made to a proxy.
So it seems that the server you send this request to is not a proxy but probably the specified server itself.
Using the absolute URI in the request line in a request sent to the specified server is only valid in HTTP 1.1:
To allow for transition to absoluteURIs in all requests in future versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI form in requests, even though HTTP/1.1 clients will only generate them in requests to proxies.