Fix bug combined expression connectors

[Abdeldjalil-H]

This PR changes expressions.Connector values to be explicit strings. This is because the operator module does not have a function called div (it is truediv).

Motivation and Context

We use enum values instead of names to get the operator function from operator module.

How Has This Been Tested?

Running existing tests.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 12106864031

Details

• 8 of 8 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.282%

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Totals
Change from base Build 12072061244:	0.0%
Covered Lines:	6305
Relevant Lines:	6877

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💛 - Coveralls

[henadzit]

Nice catch!