allow multiple OGC_SRS synonyms for WebMaps

[raphaelquast]

Rationale

Another fix I noticed related to WebMap services.
(@dopplershift I know it would have been better to already add this in #2136 but you were too fast in merging 😅 )

At the moment the identification of OGC_SRS identifiers is done via the
_CRS_TO_OGC_SRS dict which is structured as follows: {projection : "EPSG----"}

This however creates a problem if multiple codes exist, which is especially true for the most commonly used WebMercator projection, e.g:

• 900913 (outdated but still sometimes used and at the moment the only hardcoded synonym for ccrs.GOOGLE_MERCATOR)
• 3857 (the actual epsg code for the projection (see osm-wiki)

This pull-request changes the definition for _CRS_TO_OGC_SRS to use lists (so that multiple synonyms can be provided), and implements the necessary changes in the functions that use _CRS_TO_OGC_SRS.

In addition the check is now done based on lower-case strings to avoid issues with WebMaps that use epsg:--- instead of EPSG:----.

Implications

• WebMap services provided with a SRS identifiers EPSG:3857 now work as expected.
• WebMaps using lower-case SRS identifiers now also work as expected (e.g. "epsg:----")