add unit tests against RFC 7033
extract hostname discovery tests as separate parametrized class
refactor mock HttpResponse factory
prepare Webfinger Basic tests to be extendable for WebFist tests
add WebFist tests

Added unit tests testing the following:
- basic webfinger functionality against WebFist fallback code
- DKIM validation
Fix not-responding-twice HttpResponse mock
Improve host name discovery and sending proper Accept header
Add error handling

Now client throws ResourceNotFoundException if WebFinger resource cannot be looked up and throws more generic WebfingerClientException on the rest of the failures (reason included)
WebFinger misc tests
Add support for 308 redirect and remade redirect unit tests

Now HttpClient uses custom RedirectStrategy allowing 308 redirects. Redirect unit tests remade to be rather functional tests using Mock Server as endpoint
Fix negative redirect test
Fix 4xx and 5xx response handling by WebFist
Add links filtering through optional rel argument.

Client provides WebFinger server with rel parameters, so that links may be filtered on server. Additionally, the client filters links itself (as server-side link filtering is optional according to RFC 7033)
add DKIM signature validation for WebFist fallback

After successful DKIM validation client checks that sender in "FROM" header matches the target user email
make WebFist fallback to work only for acct: resources
make WebFist fallback to work only for acct: resources
fix filename capitalization issues
accept only WebFist responses with proof signed by the domain of the user

Reject lookup if user has address bob@mydomain.com and WebFist proof for that email is not signed by mydomain, but rather by different domain
remove whitespaces from mock DKIM signature
add new host discovery test cases: empty URI and URI with port