fix(http): normalize header keys to be case insensitive in RequestBuilder

[pxDot]

Summary of changes

Make header keys case insensitive in RequestBuilder.
This still has the potential issue of variable name casing not being respected though, so I'm not sure if there maybe is a better way to solve this. Maybe we need to also lowercase the keys while resolving the headers for the controller.

async controllerFunction(authorization: HttpHeader<string>) {...} // works even if sending a header with `Authorization` key via TestingFacade
async controllerFunction(Authorization: HttpHeader<string>) {...} // does not work (`Validation error:\nAuthorization(type): No value given`) as we are now lowercasing the keys

Potentially closes: #653

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