Use default init allocator for vectors

This speeds up our case as we don't need to zero the memory first.

src/WebResponseImpl.h

@@ -27,6 +27,7 @@
 #undef max
 #endif
 #include <vector>
+#include "default_init_allocator.h"
 // It is possible to restore these defines, but one can use _min and _max instead. Or std::min, std::max.
 
 class AsyncBasicResponse: public AsyncWebServerResponse {
@@ -46,7 +47,7 @@ class AsyncAbstractResponse: public AsyncWebServerResponse {
     // This is inefficient with vector, but if we use some other container, 
     // we won't be able to access it as contiguous array of bytes when reading from it,
     // so by gaining performance in one place, we'll lose it in another.
-    std::vector<uint8_t> _packet, _cache;
+    std::vector<uint8_t, default_init_allocator<uint8_t>> _packet, _cache;
     size_t _readDataFromCacheOrContent(uint8_t* data, const size_t len);
     size_t _fillBufferAndProcessTemplates(uint8_t* buf, size_t maxLen);
   protected:

src/default_init_allocator.h

@@ -0,0 +1,34 @@
+#pragma once
+
+// A C++ allocator that default-initializes objects
+// This principally useful for vectors of POD types, where default allocation is a no-op; so the vector can be resized "for free".
+//
+// Code shamelessly stolen from https://stackoverflow.com/a/21028912/8715474
+
+#include <memory>
+
+template <typename T, typename A = std::allocator<T>>
+class default_init_allocator : public A {
+    typedef std::allocator_traits<A> a_t;
+public:
+    // http://en.cppreference.com/w/cpp/language/using_declaration
+    using A::A; // Inherit constructors from A
+
+    template <typename U> struct rebind {
+        using other =
+            default_init_allocator
+            <  U, typename a_t::template rebind_alloc<U>  >;
+    };
+
+    template <typename U>
+    void construct(U* ptr)
+        noexcept(std::is_nothrow_default_constructible<U>::value) {
+        ::new(static_cast<void*>(ptr)) U;
+    }
+
+    template <typename U, typename...Args>
+    void construct(U* ptr, Args&&... args) {
+        a_t::construct(static_cast<A&>(*this),
+            ptr, std::forward<Args>(args)...);
+    }
+};