issues/260: Add protection against DNS rebinding attacks.

[komuw]

• DNS rebinding[2] is a method of manipulating resolution of domain names that is commonly used as a form of computer attack.
  The stdlib http.ServeMux does optionally protect against this class of attack[3][4][5]
• In our implementation, we fail the requests with a http 400 and appropriate error message if the incoming host is not equal to(or a subdomain) of the registered domain.
• We also update the middleware New function so that it panics if the given domain is invalid.

1. Fixes: check if mux is protected against dns rebinding similar to stdblib ServeMux #260
2. https://en.wikipedia.org/wiki/DNS_rebinding
3. https://github.com/golang/go/blob/go1.20.5/src/net/http/request.go#L232-L236
4. https://github.com/golang/go/blob/go1.20.5/src/net/http/server.go#L2467
5. https://github.com/golang/go/blob/go1.20.5/src/net/http/server.go#L2450-L2453

[codecov-commenter]

Codecov Report

Patch coverage: 80.72% and project coverage change: +0.26 🎉

Comparison is base (9aeb2b4) 77.72% compared to head (5ec698a) 77.98%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #276      +/-   ##
==========================================
+ Coverage   77.72%   77.98%   +0.26%     
==========================================
  Files          36       36              
  Lines        3372     3412      +40     
==========================================
+ Hits         2621     2661      +40     
  Misses        623      623              
  Partials      128      128              

Impacted Files	Coverage Δ
server/cert.go	32.15% <0.00%> (ø)
server/pprof.go	87.71% <0.00%> (ø)
server/server.go	75.10% <0.00%> (ø)
middleware/redirect.go	90.32% <95.58%> (+6.68%)	⬆️
middleware/middleware.go	100.00% <100.00%> (ø)

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