[swift5][client] improve headers order

[4brunu]

Since #7527 got merged, the global headers always override the local headers. This means that if there is a local header that you want to customise in some requests, and you also have a global request, the global request will always prevail.
This is somehow limiting, so with this PR, the local headers set in the request override the global headers.

@lightman73 @FranzMarini does this looks good to you?

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