(Combined)LoggingHandler: Add tests for http.Push support

Added two tests TestLoggingHandlerWithPush and
TestCombinedLoggingHandlerWithPush in new handlers_go18_test.go file
which is only built with Go 1.8 or higher.  Tests ensure that the
ResponseWriter passed to a handler wrapped with
handlers.LoggingHandler or handlers.CombinedLoggingHandler satisfy the
new http.Pusher interface.

handlers_go18_test.go

@@ -0,0 +1,34 @@
+// +build go1.8
+
+package handlers
+
+import (
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestLoggingHandlerWithPush(t *testing.T) {
+	handler := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		if _, ok := w.(http.Pusher); !ok {
+			t.Fatalf("%T from LoggingHandler does not satisfy http.Pusher interface when built with Go >=1.8", w)
+		}
+		w.WriteHeader(200)
+	})
+
+	logger := LoggingHandler(ioutil.Discard, handler)
+	logger.ServeHTTP(httptest.NewRecorder(), newRequest("GET", "/"))
+}
+
+func TestCombinedLoggingHandlerWithPush(t *testing.T) {
+	handler := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		if _, ok := w.(http.Pusher); !ok {
+			t.Fatalf("%T from CombinedLoggingHandler does not satisfy http.Pusher interface when built with Go >=1.8", w)
+		}
+		w.WriteHeader(200)
+	})
+
+	logger := CombinedLoggingHandler(ioutil.Discard, handler)
+	logger.ServeHTTP(httptest.NewRecorder(), newRequest("GET", "/"))
+}