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 #2833 from AndreyGeonya/master

Issue #2619 fixed
Merge pull request #2831 from kdeloach/topic/wms-uppercase

Add uppercase option for WMS TileLayer.
Merge pull request #2836 from AndreyGeonya/master

#2501, optional wheel debounce time
Merge branch 'master' of https://github.com/Leaflet/Leaflet
#1871 fixed, zoom param in setView method is not required now