Fix deserialization error for LRO which has discriminator

[msyyc]

fixes #2428

There are some new findings for legacy test when I mark initial operation as stream operation:

1. When LRO succeeds in first call, if will fail if use pipeline_response to deserialize since its context is null. One possible solution is to use pipeline_response.http_response which works both for sync and async.

2. When LRO needs poll, azure-core will set stream=false by default for poll request, so lro works as before.

3. for async initial, we need call response.load_body() for normal response otherwise the following error happens:
   

4. for async initial, we need call response.load_body() for error response otherwise the following error happens for test case test_sads_put_non_retry. However sync needn't:

5. version-tolerant and legacy use different HttpResponse type

mode	HttpResponse type	.read()	.load_body()	.stream_download()	.iter_bytes()
leagcy (sync)	azure.core.pipeline.transport.RequestsTransportResponse	No	No	Yes	No
leagcy (async)	azure.core.pipeline.transport.AioHttpTransportResponse	No	Yes	Yes	No
vesion-tolerant (sync/async)	azure.core.rest.HttpResponse/AsyncHttpResponse	Yes	No	No	Yes

[msyyc]

https://dev.azure.com/azure-sdk/29ec6040-b234-4e31-b139-33dc4287b756/_apis/build/builds/3803167/logs/534

[iscai-msft]

we can always call read, regardless of stream or not

[msyyc]

It is very strange to judge _stream when its value is True, so I think it is better to keep the logic.

[msyyc]

move to #2628