v4: Broken solidgauge chart after upgrade to latest version

Hello,

Since upgrade to v4.0.2, some of my charts generated by highcharts-export-server are broken, especially the solidegauge ones. Instead of displaying the colored circles for the values like in the v3.x version, it shows the word "Values".

I also use polar charts, and no problems with those. I don't know if it's a bug, but I couldn't find in the changelog what can cause this breaking change. No errors are reported in the log.

Any idea what could be causing this? Thanks.

#### Expected behaviour

```
npm list highcharts-export-server -g
/usr/local/lib
└── highcharts-export-server@3.1.1
```

```
curl -H "Content-Type: application/json" -X POST -d '{"type":"png","infile":{"chart":{"type":"solidgauge","height":"110%"},"credits":{"enabled":false},"title":{"text":""},"yAxis":{"min":0,"max":100,"lineWidth":0,"tickPositions":[]},"pane":{"startAngle":0,"endAngle":360,"background":[{"outerRadius":"105%","innerRadius":"85%","backgroundColor":"rgba(240,148,0,0.3)","borderWidth":0},{"outerRadius":"65%","innerRadius":"50%","backgroundColor":"rgba(39,209,190,0.3)","borderWidth":0}]},"plotOptions":{"solidgauge":{"dataLabels":{"enabled":false},"linecap":"round","stickyTracking":false,"rounded":true}},"series":[{"name":"","data":[{"color":"#f09400","radius":"105%","innerRadius":"85%","y":35}]},{"name":"","data":[{"color":"#27d1be","radius":"65%","innerRadius":"50%","y":77}]}]},"width":150,"height":150}' 127.0.0.1:7801 -o chart.png
```


#### Actual behaviour

```
npm list highcharts-export-server -g
/usr/local/lib
└── highcharts-export-server@4.0.2
```

```
curl -H "Content-Type: application/json" -X POST -d '{"type":"png","infile":{"chart":{"type":"solidgauge","height":"110%"},"credits":{"enabled":false},"title":{"text":""},"yAxis":{"min":0,"max":100,"lineWidth":0,"tickPositions":[]},"pane":{"startAngle":0,"endAngle":360,"background":[{"outerRadius":"105%","innerRadius":"85%","backgroundColor":"rgba(240,148,0,0.3)","borderWidth":0},{"outerRadius":"65%","innerRadius":"50%","backgroundColor":"rgba(39,209,190,0.3)","borderWidth":0}]},"plotOptions":{"solidgauge":{"dataLabels":{"enabled":false},"linecap":"round","stickyTracking":false,"rounded":true}},"series":[{"name":"","data":[{"color":"#f09400","radius":"105%","innerRadius":"85%","y":35}]},{"name":"","data":[{"color":"#27d1be","radius":"65%","innerRadius":"50%","y":77}]}]},"width":150,"height":150}' 127.0.0.1:7801 -o chart2.png
```


#### Reproduction steps

```
curl -H "Content-Type: application/json" -X POST -d '{"type":"png","infile":{"chart":{"type":"solidgauge","height":"110%"},"credits":{"enabled":false},"title":{"text":""},"yAxis":{"min":0,"max":100,"lineWidth":0,"tickPositions":[]},"pane":{"startAngle":0,"endAngle":360,"background":[{"outerRadius":"105%","innerRadius":"85%","backgroundColor":"rgba(240,148,0,0.3)","borderWidth":0},{"outerRadius":"65%","innerRadius":"50%","backgroundColor":"rgba(39,209,190,0.3)","borderWidth":0}]},"plotOptions":{"solidgauge":{"dataLabels":{"enabled":false},"linecap":"round","stickyTracking":false,"rounded":true}},"series":[{"name":"","data":[{"color":"#f09400","radius":"105%","innerRadius":"85%","y":35}]},{"name":"","data":[{"color":"#27d1be","radius":"65%","innerRadius":"50%","y":77}]}]},"width":150,"height":150}' 127.0.0.1:7801 -o chart2.png
```