Fix add_middleware enum comparison

[RobbeSneyders]

Fixes #1697

Because of a wrong comparison against the position Enum, middleware was not actually being added to the stack via add_middleware. This PR fixes this, adds a warning when the middleware position cannot be found, and adds a test.

[Ruwann]

Thanks a lot, @RobbeSneyders !

[coveralls]

Pull Request Test Coverage Report for Build 4788329443

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 94.042%

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Totals
Change from base Build 4774497315:	0.4%
Covered Lines:	3346
Relevant Lines:	3558

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💛 - Coveralls