fix(rpc): Update http batch request to handle singletons

[masonmcbride]

This pull request updates the HTTP batch request to handle singletons correctly.

Closes #3676

In sendRequestCommon, if the JSON response fails to unmarshal into the expected type, the function checks if the expected response type is a slice. If so, it attempts to unmarshal the JSON as a single element of that slice. When successful, it wraps the single element into a new slice and returns it, thereby gracefully handling cases where a singleton response is provided instead of an array.

[Gno2D2]

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[notJoon]

duplicated with #3678

[n2p5]

It looks like we've resolved this issue with the RPC server patch #3678 , but I still think it is worthwhile thinking about how we could make the client more forgiving as well. I'll pass long feedback line in a review.

[n2p5]

Nice first pass, please see my comments in line for a few changes.

[n2p5]

This should be expressed as a one-liner. I think nil should work, but if R needs various return types to be supported, you should be able to handle it by declaring the default value on the same line.

[n2p5]

Let's try to refactor this without reflection. Take notes from the RPC Server patch on how you could approach this. If the Slice marshalling fails, attempt to unmarshal a single response item and put that in a slice of the responses for the return.

[n2p5]

@notJoon I still think this is worth patching so that Batching responses for the client will work, even on older test nets. This follows the robustness principle (Postel's Law). If we can coax a slightly malformed response into a correct one, then it is better for the end-user experience. But, with the server patch, all testnets moving forward should be fine with the server side-only change.