WELD-942: corrected the easiest medium findbugs warnings

The bug list:
ModuleEnablement: http://goo.gl/nsb5p3
ObserverNotifier: http://goo.gl/vAMzor
DefaultLifecycleCallbackInvoker: http://goo.gl/Msyvdk
InjectionTargetFactoryImpl: http://goo.gl/dvSKPl
EventTypeAssignabilityRules: http://goo.gl/H5NrCq
BeanMethods: http://goo.gl/Vwuhcp
Beans: http://goo.gl/CSV6C9
I have already checked that the above-mentioned 6 bugs are really fixed and there is not a new bug caused by these fixes.