add ESP8266WebServer::sendContent_P and ESP8266WebServer::send_P with contentLength

[martinayotte]

• add ESP8266WebServer::sendContent_P and ESP8266WebServer::send_P with size_t contentLength argument to allow binary content to be send, such as images.
• fix in WiFiClient::write_P to use memcpy instead of memccpy allow binary content to be send, such as images.

[igrr]

Ouch, you are calling c_str() method of a temporary String instance here. Better move String declaration into a separate line.

[martinayotte]

Hi IGRR,
I've simply copied the code from Makuna, line #167, so if it need to be changed, it need to be done on both.

[igrr]

Oh okay, somehow i missed that. I'll merge everything then and do the necessary changes.

[martinayotte]

Hi again Ivan,

I hate putting my hands/arms into a wringer, especially when it is not my own code.

During my previous tests, I forgot to add PSTR("image/png") in my calls.
Adding that, I got some crashes, even in Makuna original code.
So, I conclude that original code isn't working :

prepareHeader(header, code, String(FPSTR(content_type)).c_str(), contentLength);

Changing that to what you suggest me doesn't work either :

String type = String(FPSTR(content_type));
_prepareHeader(header, code, type.c_str(), contentLength);

So, I've think the proper way is again to use memccpy_P(), but I feel it is a bit ugly :

char type[64];
memccpy_P((void_)type, (PGM_VOID_P)content_type, 0, sizeof(type));
prepareHeader(header, code, (const char )type, contentLength);

I will commit that for now, but I think it needs to be revisited...
(All this simply to avoid content_type been a PROGMEM, as I think you suggested before, it was maybe useless)
I will appreciate you help here !