* Prevent buffer overflow on received data
* pass to 7 char to avoid save to flash by SDK
* return _contentLength, avoid array reparse to know len
* Added FlashStringHelper for text and binary
* Added FlashStringHelper also to AsyncWebSocketClient
* Added PROGMEM doc
* Corrected binary was sending PSTR as text, addded len
* Server calls client method and code as asked @me-no-dev
* server calls client method and code as asked by @me-no-dev
* Changed Code presentation