Expect: 100-Continue header is duplicated in case of redirection.

See for instance in this debug output from DigestEchoCLientSSL: the second request (sent after the 307 code is processed) has the following header:

Expect: [100-Continue, 100-Continue]

while the original only had:

Expect: [100-Continue]

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[33 s, 652 ms, 450245 ns] ----- iteration 4 -----
[Server307 Server]:RedirectingServer[PID=35306,PORT=60236]:HTTP_1_1:https:SERVER:307->DigestEchoServer[PID=35306,PORT=35838]:HTTP_1_1:https:SERVER:BASIC: Got POST: /foo/
Host: [127.0.0.1:60236]
User-agent: [Java-http-client/10-internal]
Expect: [100-Continue]
Content-length: [128]
[Server307 Server]:RedirectingServer[PID=35306,PORT=60236]:HTTP_1_1:https:SERVER:307->DigestEchoServer[PID=35306,PORT=35838]:HTTP_1_1:https:SERVER:BASIC: Redirecting to server
[Server307 Server]:RedirectingServer[PID=35306,PORT=60236]:HTTP_1_1:https:SERVER:307->DigestEchoServer[PID=35306,PORT=35838]:HTTP_1_1:https:SERVER:BASIC: Sent back 307 Location: https://127.0.0.1:35838/
[Basic Server Filter]:DigestEchoServer[PID=35306,PORT=35838]:HTTP_1_1:https:SERVER:BASIC: Got POST: /
Host: [127.0.0.1:35838]
User-agent: [Java-http-client/10-internal]
Authorization: [Basic YXJ0aHVyOmRlbnQ=]
Expect: [100-Continue, 100-Continue]
Content-length: [128]
[Basic Server Filter]:DigestEchoServer[PID=35306,PORT=35838]:HTTP_1_1:https:SERVER:BASIC: processing Basic YXJ0aHVyOmRlbnQ=
Scheme enabled for [SERVER307, BASIC, HTTPS]: Expecting 200, response is: (POST https://127.0.0.1:60236/foo/) 200
[33 s, 718 ms, 817890 ns] ----- iteration 5 -----