fix: browser fetch can support streaming request bodies

[tshedor]

This is a bug fix related to client-streaming requests.

Invoking a MethodKind.ClientStreaming method using a connect or grpc-web transport immediately throws the error "The fetch API does not support streaming request bodies."

This, however, appears to be a dated catch. Streaming as a request body is supported by most modern browsers using the default fetch.

This PR does a very light feature check to ensure backwards compatibility instead of checking against the method's kind.

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[tshedor]

This may be a half-baked duplicate of #670 , and if that's the case, I'm sorry for adding to the maintainer's administrative work

[timostamm]

Hey Tim, thanks for bringing it up! Unfortunately, only Chrome supports streaming request bodies yet, but it's not obvious from the documentation out there.