[#63] Adds default value for finish_span_on_close in scope manager activation

[jcchavezs]

Short description of what this PR does:

• Adds a default value for $finishSpanOnClose in ScopeManager::activate.

Checklist

• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the [Contribution Guide] and my PR follows them.
• [x] I updated my branch with the master branch.
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added in line documentation to the code I modified

Closes #63