arg_read_size doesn't understand GenericIOBuffer

[c42f]

Through a weird series of events, I've discovered that

Downloads.arg_read_size(IOBuffer(codeunits("hi"))) === nothing

(This causes us not to call set_upload_size(), which in turn causes libcurl to set Transfer-Encoding: chunked, which in turn causes PutObject to S3 to fail when input is of type Base.GenericIOBuffer{Base.CodeUnits{UInt8, String}}.

I guess we should define arg_read_size(::GenericIOBuffer)?

[StefanKarpinski]

For IOBuffer, do we know how big the upload will be? Calling io.size seems too low level.

[c42f]

I looked this up. It looks like bytesavailable is the correct function.