Manually creating a Plan Branch throws error on SimpleActionParametersMap

Issue Summary

When creating a branch manually, we call SimpleActionParametersMap.put() which since 2017 has thrown WARNs

2019-03-22 18:10:50,737 WARN [http-nio-8085-exec-4] [SimpleActionParametersMap] Mutation of action parameter map: chainKey your plugin needs to be fixed
java.lang.Throwable
...
2019-03-22 18:10:50,756 WARN [http-nio-8085-exec-4] [SimpleActionParametersMap] Mutation of action parameter map: branchName your plugin needs to be fixed
java.lang.Throwable
...

Steps to Reproduce

1. Create a Plan PROJECT-PLAN
2. Manually create a branch

Expected Results

Branch is created as expected, with no warnings

Actual Results

The branch is created, but the below WARN is thrown in the atlassian-bamboo.log file:

2019-03-22 18:10:50,737 WARN [http-nio-8085-exec-4] [SimpleActionParametersMap] Mutation of action parameter map: chainKey your plugin needs to be fixed
java.lang.Throwable
...
2019-03-22 18:10:50,756 WARN [http-nio-8085-exec-4] [SimpleActionParametersMap] Mutation of action parameter map: branchName your plugin needs to be fixed
java.lang.Throwable

Workaround

As this is only logging, and has no other effect, the warnings can be suppressed by setting the log level for com.atlassian.bamboo.collections.SimpleActionParametersMap to ERROR, either:

• through the UI in Administration >> Overview >> Log settings; OR
• adding the following to <bamboo-install>/atlassian-bamboo/WEB-INF/classes/log4j.properties

  log4j.logger.com.atlassian.bamboo.collections.SimpleActionParametersMap=ERROR
  

  editing log levels through log4j.properties requires a Bamboo restart