Static : add support for HEAD requests (also avoid useless filesystem call for non HEAD/GET requests)
Static : fix and improve tests.

- The tests were previously always passing (response.Code is initialised to http.StatusOK, without a router it won't be modified even if the file does not exists).
- Also check response.Body for HEAD and GET requests.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	static_test.go