WebMapServiceImageryProvider clobbers srs option

If, theoretically, one were using a different SRS, such as one for the moon, and wanted to pass that through to a WMS via the WebMapServiceImageryProvider, there is currently no way to do so as this is hardcoded as follows:

// Use SRS or CRS based on the WMS version.
if (parseFloat(resource.queryParameters.version) >= 1.3) {
// Use CRS with 1.3.0 and going forward.
// For GeographicTilingScheme, use CRS:84 vice EPSG:4326 to specify lon, lat (x, y) ordering for
// bbox requests.
parameters.crs = options.tilingScheme instanceof WebMercatorTilingScheme ? 'EPSG:3857' : 'CRS:84';
} else {
// SRS for WMS 1.1.0 or 1.1.1.
parameters.srs = options.tilingScheme instanceof WebMercatorTilingScheme ? 'EPSG:3857' : 'EPSG:4326';
}

It would be a simple thing to check for the srs parameter in the options, and use that instead.

Start by locating the WebMapServiceImageryProvider entry point and inspect the shown WMS-version branch and its options/query-parameter flow. Verify how an explicitly supplied SRS is handled for different WMS versions, then run the relevant provider tests and confirm the request preserves that option.