JSON Encoder: Don't panic if EncodeLevel unset (#1058)

The JSON Encoder currently panics during EncodeEntry if the EncodeLevel
option is unset.
Fix this by adding a nil check for it.
This mirrors the behavior of the Console Encoder.

zapcore/json_encoder.go

@@ -364,7 +364,7 @@ func (enc *jsonEncoder) EncodeEntry(ent Entry, fields []Field) (*buffer.Buffer,
 	final := enc.clone()
 	final.buf.AppendByte('{')
 
-	if final.LevelKey != "" {
+	if final.LevelKey != "" && final.EncodeLevel != nil {
 		final.addKey(final.LevelKey)
 		cur := final.buf.Len()
 		final.EncodeLevel(ent.Level, final)

zapcore/json_encoder_test.go

@@ -136,6 +136,35 @@ func TestJSONEncodeEntry(t *testing.T) {
 	}
 }
 
+func TestNoEncodeLevelSupplied(t *testing.T) {
+	enc := zapcore.NewJSONEncoder(zapcore.EncoderConfig{
+		MessageKey:     "M",
+		LevelKey:       "L",
+		TimeKey:        "T",
+		NameKey:        "N",
+		CallerKey:      "C",
+		FunctionKey:    "F",
+		StacktraceKey:  "S",
+		EncodeTime:     zapcore.ISO8601TimeEncoder,
+		EncodeDuration: zapcore.SecondsDurationEncoder,
+		EncodeCaller:   zapcore.ShortCallerEncoder,
+	})
+
+	ent := zapcore.Entry{
+		Level:      zapcore.InfoLevel,
+		Time:       time.Date(2018, 6, 19, 16, 33, 42, 99, time.UTC),
+		LoggerName: "bob",
+		Message:    "lob law",
+	}
+
+	fields := []zapcore.Field{
+		zap.Int("answer", 42),
+	}
+
+	_, err := enc.EncodeEntry(ent, fields)
+	assert.NoError(t, err, "Unexpected JSON encoding error.")
+}
+
 func TestJSONEmptyConfig(t *testing.T) {
 	tests := []struct {
 		name     string