The request header is partially missing

[w8350196]

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• [x] I have searched for a similar issue in the project and found none

Issue Info

Info	Value
Platform Name	flutter
Platform Version
Dio Version	4.0.1
Android Studio / Xcode Version	e.g. Android Studio 3.3.2 / Xcode 10.2.1
Repro rate	e.g. all the time (100%) / sometimes x% / only once
Repro with our demo prj	e.g. does it happen with our demo project?
Demo project link	e.g. link to a demo project that highlights the issue

Issue Description and Steps

We use Dio to send the network request and we will put some data in the request header. occasionally there is some data loss in the request header acquired by the service side. Has anyone experienced this problem?