Fix CORS error with attachments

[vmx]

When attachments were returned (?attachments=true) while CORS was
enabled and used, an error occured. The reason for this error
were headers that were encoded as binaries instead of lists. String
operations on binaries throw errors.

This commit fixes COUCHDB-1689.

[rnewson]

We prefer binaries to lists, is there a fix for this that goes the other way?

[vmx]

@rnewson That's not true. When headers are used with CORS they always only work with lists, not binaries.

Of course the code base could be changed so that all headers use binaries and CORS always converts the binaries to lists. But that's not the point of this patch :)

[rnewson]

I understand the point of the patch, I'm just saying that if we are trying to fix this mismatch we prefer binaries over lists. If that's a much uglier patch (or even impossible) fair enough.

[vmx]

Now I got the point. I fear to break something with changing the headers from lists to binaries. From the source it looked like lists were preferred. So I would go with this patch and make it properly once we the whole HTTP layer gets cleanup/rewritten.

[rnewson]

sounds good.

[rnewson]

merged, thanks! please to close pull, yah?

[vmx]

Thanks for merging so quickly.