This issue would likely be triggered when automatically
merging multiple requests into a single one while having
their size exceed 1044456 bytes like SaveFilePartRequest.
This commit avoids such issue by keeping track of the
current size, and if it exceeds the limit, avoid merge.
This simplifies the flow instead of having separate request/body
attributes, and also means that BinaryReader.tgread_object() can
be used without so many special cases.
