Fix adaptor doesn't convert header values correctly

[reedchan7]

Fix(adaptor): Fixed an issue where the adapter did not convert all values of the header successfully when converting the header.

Change Header.Set() to Header.Add() to support header that has multiple values such as Accept

Relative reference: gofiber/fiber#1329

[erikdubbelboer]

Thanks!

[ReneWerner87]

@erikdubbelboer with this change a bug has crept in

by doing the Add instead of Set, the ContentType is no longer set and the default ContentType is used, although it was already set by the Add.

gofiber/fiber#1351 (comment)
gofiber/fiber#1351 (comment)
gofiber/fiber#1351 (comment)

We just use
https://github.com/gofiber/fiber/blob/master/middleware/pprof/pprof.go#L13

and our test is broken
https://github.com/gofiber/fiber/blob/master/middleware/pprof/pprof_test.go#L43

although the content type "text/html; charset=utf-8" was set inside
https://golang.org/src/net/http/pprof/pprof.go?s=11637:11687#L377

[ReneWerner87]

@juan-chan can you look at this again and make a new correction to the initial problem with an eye on this resulting error?

[erikdubbelboer]

@ReneWerner87 a fix is in progress here #1036 and will be merged next week of it doesn't break for anyone.

[ReneWerner87]

Thanks for the quick response and the link