[Swift5][URLSession] Fix handling of customHeaders between retries

[lightman73]

While trying to implement @4brunu suggestion for token refresh ( #5462 (comment) ), with an access token configured in customHeaders with key "Authorization", I noticed that after changing the value in the customHeaders in the TokenRefresher, and then calling shouldRetry(true), the following call would still use the old access token value.
Looking into the issue I noticed that the new value (set via MyAPI.customHeaders.updateValue(accessToken, forKey: "Authorization") before calling shouldRetry(true) was being overwritten with the old one stored in self.headers in the createURLRequest function defined in URLSessionImplementations.swift.
The proposed change creates the new headers for the modifiedRequest by adding the customHeaders last, so that they can overwrite the keys stored in self.headers, thus fixing the problem.

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@jgavris @ehyche @Edubits @jaz-ah @4brunu

[4brunu]

Looks good to me 👍

[wing328]

@lightman73 thanks for the PR. Can you merge the latest master of the official repo into your branch and update the Swift petstore samples as well?

[lightman73]

@lightman73 thanks for the PR. Can you merge the latest master of the official repo into your branch and update the Swift petstore samples as well?

Done ! I fixed all the URLSessionImplementations.swift in the PetStore samples to reflect the change in the mustache.

Just let me know if there is anything else that needs to be done :)