doc: add desc about gzip and http dump

Fix #3872

docs/content/docs.md

@@ -1378,7 +1378,14 @@ Write CPU profile to file.  This can be analysed with `go tool pprof`.
 #### --dump flag,flag,flag ####
 
 The `--dump` flag takes a comma separated list of flags to dump info
-about.  These are:
+about.
+
+Note that some headers including `Accept-Encoding` as shown may not 
+be correct in the request and the response may not show `Content-Encoding`
+if the go standard libraries auto gzip encoding was in effect. In this case 
+the body of the request will be gunzipped before showing it.
+
+The available flags are:
 
 #### --dump headers ####
 

fs/fshttp/http.go

@@ -180,6 +180,13 @@ func NewTransportCustom(ci *fs.ConfigInfo, customize func(*http.Transport)) http
 	t.IdleConnTimeout = 60 * time.Second
 	t.ExpectContinueTimeout = ci.ExpectContinueTimeout
 
+	if ci.Dump&(fs.DumpHeaders|fs.DumpBodies|fs.DumpAuth|fs.DumpRequests|fs.DumpResponses) != 0 {
+		fs.Debugf(nil, "You have specified to dump information. Please be noted that the "+
+			"Accept-Encoding as shown may not be correct in the request and the response may not show "+
+			"Content-Encoding if the go standard libraries auto gzip encoding was in effect. In this case"+
+			" the body of the request will be gunzipped before showing it.")
+	}
+
 	// customize the transport if required
 	if customize != nil {
 		customize(t)