inconsistent behavior for duplicate key headers

[iscai-msft]

I've noticed different behavior for request headers with duplicate keys between calling client.send and httpx.get.

Say you have headers [("Hello": "foo"), ("Hello": "bar")]. Sending this by first creating a Request object and passing it to client.send passes "Hello": "foo, bar"

However, if I pass in these headers through httpx.get, only "Hello: bar" ends up passed to the service

I think this behavior should be consistent across these 2 different methods of sending requests, and it should combine the two different values into a comma separated string, since this way no information is being lost, and it's consistent with the response.headers behavior