[core] Leaflet_tile_layer should allow case sensitive URIs

In desktop/core/src/desktop/conf.py the leaflet_tile_layer configuration parameter is set to coerce_str_lowercase, however https://tools.ietf.org/html/rfc3986#section-6.2.2.1 specifies that URIs (with the exception of the schema and host sections) are case-sensitive.

N.B. ESRI by default is case-sensitive when specifying resources and therefore this breaks some environments.