fix(mapstr): cast inner maps to M on Clone

[mauri870]

What does this PR do?

PR #262 changed the Clone behavior to preserve the type of inner map[string]interface{} instead of casting them to M. This PR reverts this behavior since we rely on inner maps being casted to M in beats. This was not tested before, that is why we only caught it now. Additionally, add tests to ensure this behavior is preserved.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

Related issues

• Relates mapstr: fix M.Clone behaviour for arrays of M #262

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 12d0d12

cc @mauri870

[efd6]

I think this is unfortunate and a result of poorly constructed tests in libbeat/processors/add_cloud_metadata, but approving.