Incorrect buffer type leads to `MethodError` on `Base.alloc_request`

[mrufsvold]

PR #1148 changed the way requests ensure sufficient space is allocated in the IOBuffer, so readbytes! now calls Base.alloc_request. However, Base.alloc_request strictly requires IOBuffer while this readbytes! method uses a Base.GenericIOBuffer.

This leads to a MethodError in CloudStore.jl (see this issue).