Fix to handle concurrent requests correctly

[lologarithm]

What kind of change does this PR introduce?

Copies headers from the transport instead of using them directly. This still probably isn't the "best" solution (we don't need a custom transport for this) but at least its threadsafe now.

What is the current behavior?

I was hitting this issue in my own code when trying to upload multiple files concurrently.

Someone reported it here: https://github.com/supabase-community/storage-go/issues/27

What is the new behavior?

It no longer crashes.

Additional context

N/A

[lologarithm]

Are there any maintainers active?