Fix sending headers in #send_P(int, PGM_P, PGM_P, size_t) (#6881)

The method #send(int, char*, char*[, size_t])) is a virtual method which
calculates the size of the content then calls #send_P(int, PGM_P, PGM_P,
size_t). This particular implementation of #send_P differs from the other
implementations of #send and #send_P in that it uses #sendContent for
headers and always calls #sendContent_P for contents even when the
contents is not specified.

The method #sendContent is intended for body and prepends the chunksize
in chunk mode but this breaks the HTTP protocol which does not expect a
chunksize prior to the headers.

Fix is simply to do the same thing as all the other methods - call
_currentClient.write and only call #sendContent_P if there is content to
send.

Author: jennytoo

libraries/ESP8266WebServer/src/ESP8266WebServer-impl.h

@@ -464,8 +464,10 @@ void ESP8266WebServerTemplate<ServerType>::send_P(int code, PGM_P content_type,
     char type[64];
     memccpy_P((void*)type, (PGM_VOID_P)content_type, 0, sizeof(type));
     _prepareHeader(header, code, (const char* )type, contentLength);
-    sendContent(header);
-    sendContent_P(content, contentLength);
+    _currentClient.write((const uint8_t *)header.c_str(), header.length());
+    if (contentLength) {
+      sendContent_P(content, contentLength);
+    }
 }
 
 template <typename ServerType>