Get and Set global filterMode using the /realm-config/agents/J2EEAgent endpoint does not work as expected

Bug description

A GET on the /realm-config/agents/J2EEAgent endpoint does not work as expected where when setting the global 'Agent Filter Mode' option to a value other than ALL then the value is not returned by the endpoint.

Conversely when using PUT on the same endpoint to update this does not allow the setting of a global value other than ALL.

How to reproduce the issue (GET)

1). Create a J2EE agent profile

2). Use the API explorer to send the following request or send manually:

curl -X GET --header 'Accept: application/json' 'http://openam.example.com:8080/AM6/json/realm-config/agents/J2EEAgent/testj2ee

3). The response contains:

 "filterMode": {
      "inherited": false,
      "value": {
        "": "ALL"
      }
    },

4). Using the admin UI adjust the agent profile so that the Agent Filter Mode is set to, for example, global SSO_ONLY by leaving the Map Key empty and setting the Map Value to SSO_ONLY (remove the default ALL entry).

5). Execute the curl request again and note the results:

 "filterMode": {
      "inherited": false,
      "value": {}
    },

Expected behaviour

 "filterMode": {
      "inherited": false,
      "value": {
        "": "SSO_ONLY"
      }
    },

Current behaviour

"filterMode": {
      "inherited": false,
      "value": {}
    },

How to reproduce the issue (PUT to update)

1). Using the admin UI set the global value to SSO_ONLY and then use the API explorer to send the following request or send manually to set it back to global ALL:

curl -X PUT --header 'Content-Type: application/json' --header 'Accept: application/json' -d '{ \
   "globalJ2EEAgentConfig": { \
    \
     "filterMode": { \
       "inherited": false, \
       "value": {"": "ALL"} \
     } \
 } \
 }' 'http://openam.example.com:8080/AM6/json/realm-config/agents/J2EEAgent/testj2ee

This succeeds as expected.

2). Now try to set this to global SSO_ONLY by sending:

"filterMode": { \
       "inherited": false, \
       "value": {"": "SSO_ONLY"} \
     }

This fails with the following response rather than overwriting ALL:

{
  "code": 400,
  "reason": "Bad Request",
  "message": "Data validation failed for the attribute, com.sun.identity.agents.config.filter.mode"
}

3). Verify using the admin UI that the value is still set to ALL

4). Using the admin UI try to set it to SSO_ONLY and note that this now fails with the following error:

Plug-in com.sun.identity.idm.plugins.internal.AgentsRepo: Error while setting attributes for agentonly=testj2ee

Note 6.0 SNAPSHOT version tested was 16 November 2017