Potential fix for #175

What has been observed is that updated values are not being sent, indicating a
potential client-side caching issue. The patch presented sets up cache-busting
headers for all GET requests to the admin API. We previously had "Cache-Control:
no-cache", but this is more comprehensive.
Fixed issues with header generation

- Expires header expects a DateTime object, and if it gets a scalar,
  creates a DateTime from it. As such, you cannot set `Expires: 0`.
  Modified to send a GenericHeader instance to bypass that
  functionality.
Also trigger for HEAD requests
Merge branch 'hotfix/http-cache-disable'

Close #218