Avoid modifying the DefaultClient's Transport

Signed-off-by: Nghia Tran <tcnghia@gmail.com>

v2/protocol/http/protocol.go

@@ -102,7 +102,10 @@ func New(opts ...Option) (*Protocol, error) {
 	}
 
 	if p.Client == nil {
-		p.Client = http.DefaultClient
+		// This is how http.DefaultClient is initialized. We do not just use
+		// that because when WithTransport is used, it will change the client's
+		// transport, which would cause that transport to be used process-wide.
+		p.Client = &http.Client{}
 	}
 
 	if p.roundTripper != nil {