client: use constants for http methods

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/network_create_test.go

@@ -37,7 +37,7 @@ func TestNetworkCreate(t *testing.T) {
 				return nil, fmt.Errorf("Expected URL '%s', got '%s'", expectedURL, req.URL)
 			}
 
-			if req.Method != "POST" {
+			if req.Method != http.MethodPost {
 				return nil, fmt.Errorf("expected POST method, got %s", req.Method)
 			}