Return UNSPECIFIED filter category when not implemented by ZuulFilter

[gavinbunney]

Rather than throw an exception when a category is not defined on a ZuulFilter subclass, return an UNSPECIFIED category instead. There are cases when filters are instantiated per request (like ProxyEndpoint) or in anonymous classes (like out static response handler), so this provides a better default safe-guard when the Filter annotation is not in use.