add full support HEAD requests

[RoboSparrow]

How much would it take to add HEAD support for the missing 3 xAPI endpoints below? We ill support them in the next release, so if it is too complicated we might just leave it as it is

test:

node test node test -d test/710-http-head

  HEAD /about
    ✓ MUST accept HEAD request

  HEAD /activities/profile
    ✓ MUST accept HEAD request

  HEAD /activities/state
    1) MUST accept HEAD request

  HEAD /activities
    2) MUST accept HEAD request

  HEAD /agents/profile
    ✓ MUST accept HEAD request

  HEAD /agents
    3) MUST accept HEAD request

  HEAD /statements
    ✓ MUST accept HEAD request

  4 passing (4s)
  3 failing

  1) HEAD /activities/state MUST accept HEAD request:
      AssertionError: response status: 200: expected 400 to equal 200

  2) HEAD /activities MUST accept HEAD request:
      AssertionError: response status: 200: expected 404 to equal 200

  3) HEAD /agents MUST accept HEAD request:
      AssertionError: response status: 200: expected 500 to equal 200

[RoboSparrow]

related Brightcookie/lxHive-Internal#142