Don't retain write buffer when chunking responses

ListBlobs and ListAllBlobs are currently returning corrupted blobs when the blob is large enough to require chunking. This is because the chunkers are retaining the writers buffer. As the writer performs the next write, it overwrites the exisiting data in the buffer. This leads to the data in the previous chunks being overwritten with the latest chunks data. Fix the issue by copying the data.

Closes #6118 (closed)