Description of problem:
RHQ Agent operation without parameters (update all plugins) fails when operation with parameters (get plugin info) was previously selected.

Version-Release number of selected component (if applicable):
Version: 3.1.1.ER3
Build Number: 37108ca:67c6de8

How reproducible:
Always

Steps to Reproduce:
1. RHQ agent is imported to inventory
2. navigate to agent's operations tab
3. Create New Operation Schedule (click on new button)
4. select 'get plugin info' operation from combo box (just select it, no other action)
5. select 'update all plugins' and click on shedule button
6. check operation result
  
Actual results:
java.lang.RuntimeException: Failed to invoke operation [updatePlugins]
	at org.rhq.plugins.agent.AgentServerComponent.invokeOperation(AgentServerComponent.java:207)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:616)
	at org.rhq.core.pc.inventory.ResourceContainer$ComponentInvocationThread.call(ResourceContainer.java:634)
	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
	at java.util.concurrent.FutureTask.run(FutureTask.java:166)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
	at java.lang.Thread.run(Thread.java:679)
Caused by: java.lang.IllegalArgumentException: Operation [updatePlugins] does not support params
	at org.rhq.plugins.agent.AgentServerComponent.invokeOperation(AgentServerComponent.java:203)
	... 10 more

Expected results:
no errors

Additional info:
next invocation of 'update all plugins' operation is successful