add SetHeaderModificator (ie to set the Server header).
fix a bug in the response modificator that skipped non-body modificators fully.
refactor ResponseModificator, get rid of ExecutionContext and Transferer in the method signature.
refactored Query class to allow for Query.fromURI.
Crash the ModifyingProxy actor if modification or transfer fails.
add portlet resolver.
Use default port for scheme if port in uri is unspecified.