Make params map immutable

[nre-ableton]

The params map is also immutable on a real Jenkins instance, so code
shouldn't be allowed to modify it directly here, either.

Fixes #528.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[gulyas-istvan-netlock-hu]

Hello @nre-ableton ,
In declarative pipelines the choices clause always throws exception, and impossible to test production code which working on prod Jenkins :(

The clause without parameters unusable there, because the choice parameters are immutable later.

The feature is not simulating Jenkins now :(

java.lang.UnsupportedOperationException
	at java.base/java.util.Collections$UnmodifiableMap.put(Collections.java:1457)
...
	at com.lesfurets.jenkins.unit.declarative.ParametersDeclaration.setParams(ParametersDeclaration.groovy:9)
	at com.lesfurets.jenkins.unit.declarative.ParametersDeclaration$setParams.callCurrent(Unknown Source)
...
	at com.lesfurets.jenkins.unit.declarative.ParametersDeclaration.choice(ParametersDeclaration.groovy:19)

[nre-ableton]

@gulyas-istvan-netlock-hu Can you please provide an example declarative pipeline that fails since this change?

[nre-ableton]

BTW, I think I have a fix for this, but I'd like a failing test to go along with the change.

[gulyas-istvan-netlock-hu]

A simple example I wrote into the new PR chat: #546 (comment)