Slight potential issue with wms imageLoadFunction for requestMethod: post

[Grammostola]

Describe the bug
The URL.revokeObjectURL method may never get called (it should as per mdn for mem manag)

To Reproduce
Define a map with a wms-layer with prop requestMethod: 'post'
Edit
https://github.com/origo-map/origo/blob/master/src/layer/wms.js
so that a console message appears when the event handler containing the mentioned method runs like so:

      img.addEventListener('loadend', () => {
        console.log('img loadend, going to revoke')
        URL.revokeObjectURL(url);

There will probably not be anything logged to the console

Expected behavior
'img loadend, going to revoke' for every tile I think

Additional context
The way I understand it (may be insufficiently) the HTMLImageElement doesn't seem to emit a loadend event. It has an 'onload' handler property which appears to do the job. I will whip up the smallest pr