Use default port (53) in Dnsruby::DNS when :port is nil

[smlsml]

This occurs when DNS is initialized with nil such as DNS.new(:port => nil) or when configuration is automatically detected (but there is no network connection so port is nil).

Fixes: https://github.com/alexdalitz/dnsruby/issues/153 ArgumentError (invalid port config: nil) on Dns.new() with no network connection

Also:

• Fixed tc_dns.rb#test_port test, it was nested inside of #test_searchlist and therefore was not actually running
• Added tc_dns.rb#test_port_nil test
• Turned off verbose Rack warnings (using 'rake test' was overrun by inconsequential warnings)
• Fixed tests in tc_resolv.rb and tc_resolver.rb tests - google DNS host name has changed

$ rake test
Running online tests. These tests send UDP packets - some may be lost.
If you get the odd timeout error with these tests, try running them again.
It may just be that some UDP packets got lost the first time...
-----------------------------------------------------------------------
OpenSSL not present (with full functionality) - skipping DS digest test
-----------------------------------------------------------------------
Run options: --seed 31421

# Running:

Minitest::Result | ................................................................
                 | ................................................................
                 | ................................................................
                 | ..
                 | 42.55 s
Slowest tests:
6.56 s  Minitest::Result#test_TCP_pipelining_timeout
6.23 s  Minitest::Result#test_TCP_pipelining_timeout_in_send
3.02 s  Minitest::Result#test_cache
3.00 s  Minitest::Result#test_TCP_pipelining_socket_eof
2.86 s  Minitest::Result#test_reverse_lookup
2.58 s  Minitest::Result#test_queue_timeout
Slowest suites:
42.64 s Minitest::Result

Finished in 42.675147s, 4.5460 runs/s, 40.3045 assertions/s.
194 runs, 1720 assertions, 0 failures, 0 errors, 0 skips

[coveralls]

Coverage decreased (-0.4%) to 77.6% when pulling 5299caad05326c32f733bac9db144020092bb226 on cloudvolumes:master into 06303437027149fafe78fd09249677ac55d4db34 on alexdalitz:master.

[smlsml]

$ gem install bundler YAML safe loading is not available. Please upgrade psych to a version that supports safe loading (>= 2.0). Fetching: bundler-2.0.2.gem (100%) ERROR: Error installing bundler: The last version of bundler (>= 0) to support your Ruby & RubyGems was 1.17.3. Try installing it with gem install bundler -v 1.17.3 bundler requires Ruby version >= 2.3.0. The current ruby version is 2.0.0.

[alexdalitz]

Thanks for this! Sorry about the problems with Travis - I've replaced the 2.0.0 build with 2.6.3 now, so it should be happier. As it happens, I'm about to push some changes, so I've added the changes in your PR to this. Thanks!