multipart uploads: Hack to support gsutil cp

[ekimekim]

gsutil sends an invalid multipart boundary param, which golang's mime.ParseMediaType correctly rejects.
However, the real GCS evidently does not reject this, so in order to make gsutil work we need to support it.

In particular, gsutil sends a boundary param that is quoted using single-quotes when it should be using double-quotes.
In cases where the param is definitely invalid (so we're guarenteed not to break any valid values), we replace all single-quotes with double-quotes to produce the intended meaning.

Upstream bug: GoogleCloudPlatform/gsutil#1466

Fixes #217
The above issue found this bug and worked around it, but found another problem and never submitted the fix.
It seems that in the intervening time, the other problem was fixed independently, and I can successfully gsutil cp using this code.

[ekimekim]

hm, now I think about it more, it might be safer to only do the ' -> " transform if we first try and fail to parse the mime type. right now, a boundary value like boundary="boundary='=" would do the wrong thing.

[fsouza]

Thanks for fixing this!

[fsouza]

hm, now I think about it more, it might be safer to only do the ' -> " transform if we first try and fail to parse the mime type. right now, a boundary value like boundary="boundary='=" would do the wrong thing.

Yeah, I wonder if we need something more specific, like a regexp replace that is specific to boundary='(.+)' or something like that.