fix enable_commenter functionality

[avzis]

Description

Fix enable_commenter functionality.
Fixes #1368

Now you can either:

1. call instrument with an existing engine, and enable or disable sql commenter with the enable_commenter param,
   like this:

SQLAlchemyInstrumentor().instrument(
    engine=engine,
    enable_commenter=True,
)

2. call instrument without an engine, and create it separately,
   note: it is important to import the create_engine function after calling instrument():

SQLAlchemyInstrumentor().instrument(enable_commenter=True)
from sqlalchemy import create_engine
engine = create_engine("sqlite:///:memory:")

Type of change

Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Now the tests are covering all 4 cases:
sqlcommenter enabled and disabled, both when instrumenting with engine, and when creating the engine after instrumentation.

• test_sqlcommenter_disabled()
• test_sqlcommenter_enabled()
• test_sqlcommenter_enabled_create_engine_after_instrumentation()
• test_sqlcommenter_disabled_create_engine_after_instrumentation()

Does This PR Require a Core Repo Change?

No.

[osherv]

I think the default parameters here are redundant, since they are always passed from the callee funtions