[JUNIT] added one test case for ConnectionPool class

Also updated the JavaDoc of the get method.

Change-Id: Ib981fc180652fa0f5363257b6ea85e2cf5e482aa
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1640
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>