fix: keep http.Hijacker when wrapping a response writer

[knQzx]

NewWrapResponseWriter only returns a wrapper with a Hijack method when the underlying writer implements http.Flusher, http.Hijacker and io.ReaderFrom at once. a writer that supports flushing and hijacking but not the io.ReaderFrom optimization falls through to flushWriter, which has no Hijack, so protocol upgrades behind sentryhttp and sentrynegroni fail with "ResponseWriter is not an http.Hijacker"

losing functionality because an optimization is missing looks unintended: the upstream chi wrapper this file is derived from has flushHijackWriter and hijackWriter fallbacks for exactly these combinations, and they were dropped in this copy. the CHANGELOG also declares "Add support for the http.Hijacker interface", so hijacking is meant to survive the wrapper

this restores both fallbacks and adds tests for the three hijackable combinations