Allow "default" headers to be defined for all responses. (#230)

* Add DefaultHeaders singleton.

* Add documentation for DefaultHeaders.

src/ESPAsyncWebServer.h

@@ -414,6 +414,31 @@ class AsyncWebServer {
     void _rewriteRequest(AsyncWebServerRequest *request);
 };
 
+class DefaultHeaders {
+  using headers_t = LinkedList<AsyncWebHeader *>;
+  headers_t _headers;
+  
+  DefaultHeaders()
+  :_headers(headers_t([](AsyncWebHeader *h){ delete h; }))
+  {}
+public:
+  using ConstIterator = headers_t::ConstIterator;
+
+  void addHeader(const String& name, const String& value){
+    _headers.add(new AsyncWebHeader(name, value));
+  }  
+  
+  ConstIterator begin() const { return _headers.begin(); }
+  ConstIterator end() const { return _headers.end(); }
+
+  DefaultHeaders(DefaultHeaders const &) = delete;
+  DefaultHeaders &operator=(DefaultHeaders const &) = delete;
+  static DefaultHeaders &Instance() {
+    static DefaultHeaders instance;
+    return instance;
+  }
+};
+
 #include "WebResponseImpl.h"
 #include "WebHandlerImpl.h"
 #include "AsyncWebSocket.h"

src/WebResponses.cpp

@@ -94,7 +94,11 @@ AsyncWebServerResponse::AsyncWebServerResponse()
   , _ackedLength(0)
   , _writtenLength(0)
   , _state(RESPONSE_SETUP)
-{}
+{
+  for(auto header: DefaultHeaders::Instance()) {
+    _headers.add(new AsyncWebHeader(header->name(), header->value()));
+  }
+}
 
 AsyncWebServerResponse::~AsyncWebServerResponse(){
   _headers.free();