Buffer.indexOf(ByteString) accepts negative indices

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While Source.indexOf(ByteString) throws IllegalArgumentException on attempts to supply negative indices, Buffer's overload accepts them and return some results.

Expected behavior: Buffer.indexOf(ByteString(...), -1) should throw IAE.

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Expected behavior: Buffer.indexOf(ByteString(...), -1) should throw IAE.

If we take CharSequence.indexOf behavior as a reference, negative indices should be treated as 0.
That behavior is not the best, but the goal is providing an API that surprises as less as possible, and having similar behavior across different indexOf functions should be a step towards that goal.