gsutil uses an invalid content-type header

When you make a multipart request to copy, gsutil uses
```content-type: multipart/related; boundary='===============8541858631849441928=='``` note the single quotes. This causes parsing errors in some applications, particularly Golang's mime parser.

https://go.dev/play/p/aHwwWh_ASgs

Doing a string replace with double quotes makes it work

Start by reproducing a multipart copy request and inspect the generated Content-Type header, using the linked Go parser example to confirm the parsing failure. Locate the code that assembles the multipart boundary and update the quoting so the header is accepted by affected parsers; done means the request uses valid quoting and the copy succeeds.

Written by the indexing model from the issue text.