[wasm] HttpRequestMessage.Headers.ExpectContinue is ignored on browser

[pavelsavara]

In unit test System.Net.Http.Functional.Tests.HttpClientHandler_RemoteServerTest.PostAsync_ExpectContinue_Success(expectContinue: True, version: 1.1) on WASM in Browser

Assert.Contains() Failure Not found: "Expect": "100-continue"
   at System.Net.Http.Functional.Tests.HttpClientHandler_RemoteServerTest.PostAsync_ExpectContinue_Success(Nullable`1 expectContinue, Version version)

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Issue Details

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In unit test System.Net.Http.Functional.Tests.HttpClientHandler_RemoteServerTest.PostAsync_ExpectContinue_Success(expectContinue: True, version: 1.1) on WASM in Browser

Assert.Contains() Failure Not found: "Expect": "100-continue"
   at System.Net.Http.Functional.Tests.HttpClientHandler_RemoteServerTest.PostAsync_ExpectContinue_Success(Nullable`1 expectContinue, Version version)

Author:	pavelsavara
Assignees:	-
Labels:	arch-wasm, area-System.Net.Http
Milestone:	-

[pavelsavara]

This needs browser support, so that the HTTP client would pause after headers, before sending the HTTP payload.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Expect

But no browser does it, see
Gecko issue
Chromium issue