[ipfs/go-ipfs-files] Fix encoding of filename in Content-Disposition

[Stebalien]

We should use RFC5987 to encode the filename in the Content-Disposition header. GO will already gracefully handle line-breaks but we should still be escaping better.

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[kstuart]

@Stebalien do you mean in multiFileReader? The filename is encoded using url.QueryEscape so should be encoded according to standards.

Test added in this commit: 66d91bf (#34)

Does this ticket remain an issue?

[Stebalien]

I can't remember at this point. If that's where we set Content-Disposition, then yes.

[kstuart]

Yes, that's the only place it is set, since all values are escaped using url.QueryEscape this ticket is probably stale and could be closed.