cmd/pomerium: strip port from authentication handler

[desimone]

Go strips the port from the host only when it is trying to locate a http handler. It does not strip the port when the user adds a handler for a host that includes a port number. As a result, trying to add handlers for hosts that include the port no longer works. To fix a bug where authenticate handler's were not being found, we explicitly strip the auth host handler of any port.

Closes #104

See also:

• net/http: document and test behavior of ServeMux with ports golang/go#23351
• net/url: add tests for URLHostname golang/go#31832

Checklist:

• unit tests added
• related issues referenced
• ready for review

[codecov]

Codecov Report

Merging #106 into master will increase coverage by 0.07%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   79.47%   79.55%   +0.07%     
==========================================
  Files          31       32       +1     
  Lines        1832     1839       +7     
==========================================
+ Hits         1456     1463       +7     
  Misses        307      307              
  Partials       69       69