Fix uploads for non-seekable streams

[mwitkow]

Fixes #18

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[coveralls]

Coverage remained the same at 39.97% when pulling 4569fa5 on mwitkow-io:fix_upload_from_streamable into f92dfb0 on google:master.

[coveralls]

Coverage remained the same at 39.97% when pulling 4569fa5 on mwitkow-io:fix_upload_from_streamable into f92dfb0 on google:master.

[mwitkow]

I signed it!

[mwitkow]

Or at least our company did "Improbable Worlds Ltd"

[craigcitro]

this looks good -- just want to ask @thobrla to confirm that this is unlikely to quietly break anything for gsutil?

[thobrla]

Looks good.

Note: we use seekable() elsewhere but that only applies to IOBase objects, which is not applicable for upload streams with type 'file'.

[craigcitro]

awesome -- thanks @mwitkow-io!