pkg/httplog: change log levels for 4xx and 501 status

this changes 4xx to debug level, and 501 to warning level to
stop the log clutter when we run at info level in production.

Closes #387

Change-Id: I861cdbf8af5dbf51a35016bc42bcc23b0e079347

pkg/httplog/httplog.go

@@ -11,10 +11,10 @@ import (
 // StatusLevel takes an HTTP status and returns an appropriate log level.
 func StatusLevel(status int) zapcore.Level {
 	switch {
+	case status == 501:
+		return zap.WarnLevel
 	case status >= 500:
 		return zap.ErrorLevel
-	case status >= 400:
-		return zap.InfoLevel
 	default:
 		return zap.DebugLevel
 	}

pkg/httplog/httplog_test.go

@@ -36,11 +36,11 @@ func TestStatusLevel(t *testing.T) {
 		},
 		{
 			status:        http.StatusBadRequest,
-			expectedLevel: zap.InfoLevel,
+			expectedLevel: zap.DebugLevel,
 		},
 		{
 			status:        http.StatusNotFound,
-			expectedLevel: zap.InfoLevel,
+			expectedLevel: zap.DebugLevel,
 		},
 		{
 			status:        http.StatusInternalServerError,
@@ -50,6 +50,10 @@ func TestStatusLevel(t *testing.T) {
 			status:        http.StatusBadGateway,
 			expectedLevel: zap.ErrorLevel,
 		},
+		{
+			status:        http.StatusNotImplemented,
+			expectedLevel: zap.WarnLevel,
+		},
 	}
 	for _, tc := range testCases {
 		tc := tc