Motivation:
The class `HttpPostRequestEncoder` has minor issues:
- The `encodeNextChunkMultipart()` method contains two identical blocks of code with a difference only in the cast interfaces: `Attribute` vs `HttpData`. Because the `Attribute` is extended by `HttpData`, the block with the `Attribute` can be safely deleted.
- The `getNewMultipartDelimiter()` method contains a redundant `toLowerCase()`.
- The `addBodyFileUploads()` method throws `NPE` instead of `IllegalArgumentException`.
Modifications:
- Remove duplicated code block from `encodeNextChunkMultipart()`.
- Remove redundant `toLowerCase()` from `getNewMultipartDelimiter()`.
- Replace `NPE` with `IllegalArgumentException` in `addBodyFileUploads()`.
- Use `ObjectUtil#checkNotNull` where possible.
Result:
More correct and clean code.
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