fix errors when using zuul-core with servlet-3

I'm using zuul-core in an application that needs servlet-3.0 features. As-is, zuul-core code will throw "AbstractMethodError"s because the servlet3 container expects the RequestWrapper to implement servlet3 methods.

The changes in this pull request makes zuul-core forward-compatible with servlet-3. There will be no problems deploying this code in a servlet-2.5 environment.

Because servlet-api is included as "provided", this change will not significantly impact the dependency trees of projects that depend on zuul-core.