Allow for the header to be sent all the time, rather than just on certain response codes

[chriscpritchard]

I've created a pull request for this issue as hpkpinx wasn't sending the response header all the time (for example when HTTP authentication is turned on, it doesn't send the header until after the authentication is received and a 200 found status code is returned).

I've fixed it by adding an option of "HEADER_ALWAYS" which, when set to "1" will add the "always" option to the end of the header in the nginx config file