Add uppercase option for WMS TileLayer.

In response to issue #1751, WMS TileLayer request parameter keys were
cast to uppercase. This is a breaking change for services that are case
sensitive and expect lowercase keys.

This change adds a new option to uppercase keys, which is false by
default, to preserve backwards compatibility.
Merge pull request #2831 from kdeloach/topic/wms-uppercase

Add uppercase option for WMS TileLayer.