Avoid overriding 'server_port' if it's passed explicitly

[Madh93]

Like user and password options, I think is better not overriding server_port when server_url is passed if it was defined explicitly.

Right now:

JenkinsApi::Client.new(
  server_url: 'https://my-custom-host',
  server_port: '34567'
)
# URI.parse(@server_url).port => 443

If we want pass a custom port using server_url is necessary include it in the own server_url:

JenkinsApi::Client.new(
  server_url: 'https://my-custom-host:34567'
)
# URI.parse(@server_url).port => 34567

This way is less flexible if we need to change the port in the future. This change allow the next:

JenkinsApi::Client.new(
  server_url: 'https://my-custom-host',
  server_port: '34567'
)
# URI.parse(@server_url).port => 34567