Fix #455 and improve the normalization logic

[RobertoPrevato]

• Removes the unused "active" property defined in the Response class.
• Fixes #455, reported by @Klavionik. This error caused the WebSocket handler to erroneously return an instance of BlackSheep response to the underlying ASGI server, causing an error to be logged in the console.
• Update type annotations in the Application class code to be more explicit about the fact that certain methods must return None (return in call is used to interrupt code execution and not to return objects).
• Improve the normalization logic to not normalize the output for WebSocket requests (as ASGI servers do not allow controlling the response for WebSocket handshake requests).
• Improve the normalization logic to not normalize request handlers that are valid as they are, as asynchronous functions with a single parameter annotated as Request or WebSocket.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (8d9321c) 97.49% compared to head (37dc56a) 97.48%.

Files	Patch %	Lines
blacksheep/server/application.py	80.00%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #457 +/- ## ========================================== - Coverage 97.49% 97.48% -0.02% ========================================== Files 67 67 Lines 6355 6363 +8 ========================================== + Hits 6196 6203 +7 - Misses 159 160 +1 ```

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