Spring DataBufferUtils and BodyInserters: allow consumers of OutputStream to throw IOException

At the moment, users of DataBufferUtils.outputStreamPublisher are forced to catch and re-throw IOException whenever they actually write to the OutputStream.

Example:

DataBufferUtils.outputStreamPublisher(
    outputStream -> {
        try {
            outputStream.write(1);
            ...
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    },
new DefaultDataBufferFactory(),
Runnable::run)

This PR aims to allow users to write cleaner code, example:

DataBufferUtils.outputStreamPublisher(
    outputStream -> {
        outputStream.write(1);
        ...
    },
new DefaultDataBufferFactory(),
Runnable::run)

As exceptions are already handled nicely under the hoods.

Same concept for BodyInserters.

Comment From: pivotal-cla

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Comment From: pivotal-cla

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