Keep session in HTTPClient in UrlInvoker

[tomasbjerre]

Problem identified when integrating with Jenkins to get crumb and then use the crumb in a later invocation. #349

https://github.com/tomasbjerre/pull-request-notifier-for-bitbucket/blob/master/src/main/java/se/bjurr/prnfb/http/UrlInvoker.java

[kentgitbub]

I wish I was fluent in JAVA. I would have created a PR and fixed it. I hope that someone else is if Tomas has to much on his plate.