http api Identify with HEAD request

[travisperson]

I wasn't able to find a really easy way to determine if the service running on port 5001 was the ipfs daemon. It might be beneficial to have the API respond to a request to HEAD requests with something to identify itself.

curl -I http://127.0.0.1:5001/ipfs/
HTTP/1.1 400 Bad Request
Date: Fri, 23 Jan 2015 06:06:42 GMT
Content-Length: 38
Content-Type: text/plain; charset=utf-8
Server: Ipfs-Daemon/0.1.7

[jbenet]

👍 on the Server header! We should use the same ipfs-agent string: go-ipfs/<version>

[jbenet]

Would be good to still:

• include the server header (should be the go-ipfs agent)

The identification can now happen by going to /version. example: http://localhost:5001/version

[whyrusleeping]

HEAD requests work now, we can close this after @Kubuxu's PR merges

[daviddias]

Security wise, it is a bad practice to announce the running service (and version) on HTTP headers or any kind of banners