[Blob][Fix]fix downloader.chunks() return chunks in different size

[xiafu-msft]

when iterate through downloader.chunks() the chunk size is different. The first chunk size is always equals to max_single_get_size while it should equal to max_chunk_get_size
This will fix #9419 and #15648

[tasherif-msft]

Great work and test LGTM :) please add the comment I mentioned in my review for that case as it will be helpful to understand the code for future devs.

[tasherif-msft]

What's the reason for calling self._get_chunk_data() in this case? as far as I understand if we hit this case (we're done iterating the first single get call and now iterating through our download chunks like we usually do) then current content should always be the same as the chunk size so we can just return self.current_content. It seems from your try-except that there is an edge case where there will be excess content - could you just put a small comment on that in the code - its not obvious to me haha

EDIT: I just realized the case is when the current content from the first get is still there but smaller than chunk size therefore we want to make sure its also included. I think adding a comment in the code would be very helpful here!
Good work thinking of all the cases