Diagnosis for test failure.

https://jenkins.ci.cloudbees.com/job/core/job/jenkins_main_trunk/850/testReport/jenkins.security/RekeySecretAdminMonitorTest/testBasicWorkflow/
com.gargoylesoftware.htmlunit.FailingHttpStatusCodeException: 400 Bad Request for http://localhost:60027//administrativeMonitor/jenkins.security.RekeySecretAdminMonitor/scan
	at com.gargoylesoftware.htmlunit.WebClient.throwFailingHttpStatusCodeExceptionIfNecessary(WebClient.java:549)
	at com.gargoylesoftware.htmlunit.WebClient.getPage(WebClient.java:333)
	at com.gargoylesoftware.htmlunit.WebClient.getPage(WebClient.java:357)
	at com.gargoylesoftware.htmlunit.html.HtmlForm.submit(HtmlForm.java:196)
	at org.jvnet.hudson.test.HudsonTestCase.submit(HudsonTestCase.java:1178)
	at jenkins.security.RekeySecretAdminMonitorTest.testBasicWorkflow(RekeySecretAdminMonitorTest.java:112)
java.lang.Exception: Invalid request submission
	at org.kohsuke.stapler.HttpResponses.error(HttpResponses.java:79)
	at jenkins.security.RekeySecretAdminMonitor.doScan(RekeySecretAdminMonitor.java:117)