credentials: don't overwrite ServerName in given config
credentials: don't overwrite ServerName in given config

The first endpoint will set the ServerName which will then be used by
the second endpoint, causing the transport to reject the second endpoint
since the server cert won't match the server name.

rakyll/grpc-go,thinkerou/grpc-go,bradfitz/grpc-go,iamqizhao/grpc-go,mpl/grpc-go,andreimatei/grpc-go,andreimatei/grpc-go,MakMukhi/grpc-go,fuchsia-mirror/third_party-grpc-grpc-go,MakMukhi/grpc-go,grpc/grpc-go,peter-edge/grpc-go,improbable-io/grpc-go,go4org/grpc,dfawley/grpc-go,thinkerou/grpc-go,menghanl/grpc-go,mpl/grpc-go,dsymonds/grpc-go,go4org/grpc,grpc/grpc-go,iamqizhao/grpc-go,rakyll/grpc-go,peter-edge/grpc-go,stanley-cheung/grpc-go,stanley-cheung/grpc-go,menghanl/grpc-go,improbable-io/grpc-go,dfawley/grpc-go,fuchsia-mirror/third_party-grpc-grpc-go,bradfitz/grpc-go,dsymonds/grpc-go