Update WMSLayer.js

Fix a bug when WMSLayer does not have "styles" parameter in options ("styles" is a required parameter).
ex : new GlobWeb.WMSLayer({ baseUrl: "http://localhost:8182/wms/wms.cgi", layers: "mola" });
Merge pull request #5 from J-Christophe/patch-1

Fix "styles" parameter for WMSLayer