Fixing multi value headers so they are serialized correctly

[AlexV525]

Picked from cfug/dio#1586

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest develop to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I am adding
• I have updated the documentation (if necessary)
• I have run the tests and they pass

This merge request fixes / refers to the following issues: There was a change in version 4.0.1 that converted request header to their string values. This breaks multi value request headers as the get serialized like this [value1,value2] instead of like this value1,value2

Pull Request Description

Change has been made to use the previous code, which worked in 4.0.0. This bug was found after upgrading from 4.0.0 to 4.0.6

[ueman]

Not really related to this PR, but I just noticed that the Response has a List<Headers> whereas the you add header to the options as a Map<String, dynamic>. It would make a lot of sense to unify that, since it would make verification of headers unified for requests and responses. It would probably also make testing easier.

[AlexV525]

Not really related to this PR, but I just noticed that the Response has a List<Headers> whereas the you add header to the options as a Map<String, dynamic>.

I only found Headers in the Response, can you indicate that?