Add RequestScopeHandler to kernel instead of SingletonScopeHandler

Ninject.Web.Common.Xml extension is supposed to add support for InRequestScope() in xml files via the "scope" attribute. 

In order to do that, it has to load RequestScopeHandler into the kernel, overriding the "Load" method of the NinjectModule class.

WebCommonXmlModule is currently loading the SingletonScopeHandler which is wrong.

More info:
https://github.com/ninject/ninject.extensions.xml/wiki/Getting-started#scope-attribute-optional