Added test to ensure NewClientWithUserAgent never modifies DefaultClient
Added test to ensure NewClientWithUserAgent never modifies DefaultClient

Signed-off-by: Brendan Dixon <e9401a09b1612f6a181940f70779b7a4ece5c52c@microsoft.com>

garimakhulbe/go-autorest,colemickens/go-autorest,Azure/go-autorest