- Introduced new registry option - customHeadersToSkipOnWekVersion which expects
to be an array of strings denoting the headers that have to be omitted from
the response in case a weak version is requested (i.e 1.x.x). The rational
behind this is the need to control cache related headers - in case 1.x.x
version is requested we would not like cache headers to be returned because
otherwise the response will be cached. Next time when someone asks for 1.x.x
it will get the cached response even though a new version may be available.
- added unit and integration tests

- also context.setHeader function was extended a bit and now includes a sanity
  check for the arguments

…and GET requests

- for GET it will put the custom headers in the HTTP response headers
- for POST it will put the custom headers in the response body
- the setCustomHeaders was moved out of component.js to get-component.js under
  the name filterCustomHeaders. It's because in case of POST request the code
  path is different and goes to component(s).js instead of component.js. Now we
  do the filtering in one and the same place.
- fixed a bug where we used to not take into account the custom headers in
  case of a unrendered component. For this reason the actual setting of
  response headers was moved from line 205 up to line 175.
- fixed the registry acceptance tests. In their former variant when the GET
  tests passes they were used to leave the customHeadersToSkipOnWeakVersion
  settings so the POST tests were starting directly with this setting on.
  That's why new before/after sections were defined to reinitialize the
  registry before the tests and then recover it after them.