Write header immediately in AbortWithStatus()

Otherwise, caller needs to invoke WriteHeaderNow himself after
AbortWithStatus(), which is error-prone.

Also modified ErrorLoggerT() such that it always writes log to response
body. Otherwise calling AbortWithStatus() will fail to write body because
c.Writer.Written() is set true by WriteHeaderNow().

recovery_test.go

@@ -39,5 +39,5 @@ func TestPanicWithAbort(t *testing.T) {
 	// RUN
 	w := performRequest(router, "GET", "/recovery")
 	// TEST
-	assert.Equal(t, w.Code, 500) // NOT SURE
+	assert.Equal(t, w.Code, 400)
 }