Force HSTS header always in Apache config

Force to send the HSTS header on successful and unsucessful response.

If Apache returns a 404 or and redirect, the HSTS header is not sent back to the client. With the condition ```always``` [1] the HSTS header is always appended.

E.g without:
```
Header add Strict-Transport-Security "max-age=15768000"
RewriteEngine On
RewriteRule ^$ /foo [R=301,L]
```
Response:
```
$ wget --spider -S http://192.168.33.62/
Spider mode enabled. Check if remote file exists.
--2014-11-17 23:15:18--  http://192.168.33.62/
Connecting to 192.168.33.62:80... connected.
HTTP request sent, awaiting response... 
  HTTP/1.1 301 Moved Permanently
  Server: Apache/2.4.7 (Ubuntu)
  Location: http://192.168.33.62/foo
  Content-Type: text/html
Location: http://192.168.33.62/foo [following]
Spider mode enabled. Check if remote file exists.
--2014-11-17 23:15:18--  http://192.168.33.62/foo
Connecting to 192.168.33.62:80... connected.
HTTP request sent, awaiting response... 
  HTTP/1.1 404 Not Found
  Server: Apache/2.4.7 (Ubuntu)
  Content-Type: text/html
Remote file does not exist -- broken link!!!
```
E.g without ```always```:
```
Header always add Strict-Transport-Security "max-age=15768000"
RewriteEngine On
RewriteRule ^$ /foo [R=301,L]
```

Response
```
$ wget --spider -S http://192.168.33.62/
Spider mode enabled. Check if remote file exists.
--2014-11-17 23:16:06--  http://192.168.33.62/
Connecting to 192.168.33.62:80... connected.
HTTP request sent, awaiting response... 
  HTTP/1.1 301 Moved Permanently
  Server: Apache/2.4.7 (Ubuntu)
  Strict-Transport-Security: max-age=15768000
  Location: http://192.168.33.62/foo
  Content-Type: text/html
Location: http://192.168.33.62/foo [following]
Spider mode enabled. Check if remote file exists.
--2014-11-17 23:16:06--  http://192.168.33.62/foo
Connecting to 192.168.33.62:80... connected.
HTTP request sent, awaiting response... 
  HTTP/1.1 404 Not Found
  Server: Apache/2.4.7 (Ubuntu)
  Strict-Transport-Security: max-age=15768000
  Content-Type: text/html
Remote file does not exist -- broken link!!!
```

[1] http://httpd.apache.org/docs/2.2/mod/mod_headers.html#header