Make Expect: 100-continue handling more robust (dotnet/corefx#24789)

Make Expect: 100-continue handling more robust (dotnet/corefx#24789)

When libcurl sends an Expect: 100-continue header, if it gets back a success error code, it may avoid sending the remaining payload and also may keep the connection open, which can confuse servers that expect to receive the fully promised payload and that don't close the connection when sending a final success status code instead of 100 continue.  To mitigate this, we simply change the default ExpectContinue == null behavior to be the equivalent of ExpectContinue == false rather than to be "do whatever the platform decides".  This also more closely aligns with WinHttpHandler and ManagedHandler, where effectively the "platform" in those cases decides that the default is disabled.

For ManagedHandler, currently we're mimicking behavior like that libcurl employed and may sometimes not send the full payload but still keep the connection open.  Instead, make sure we either always send the full payload or close the connection.

Commit migrated from https://github.com/dotnet/corefx/commit/724ddb8fc95a5b1f639cfa508a32b9a8c50c1949