Prevent flushing when writing out from the formatters.
Also make the XML formatter respect object as a return type.

Issues: MVC/1073, MVC/972, MVC/958

1072 is a followup work item to add test coverage.
Updated to use the new target framework in project.json
Changed remaining ifdefs from K10 and ASPNETCORE50
Adding support for flush points in Razor pages

Fixes #1042
Updating FunctionalTests to use HttpClient abstractions
Updating tests to use TryAddWithoutValidation