Error in question:
ArduinoHttpClient/src/HttpClient.h: In member function 'virtual void HttpClient::flush()':
ArduinoHttpClient/src/HttpClient.h:310:50: error: return-statement with a value, in function returning 'void' [-fpermissive]
Return invalidated String if memory allocation fails or content length
does not match body data length. Also, use timed reads to support
responses without a content length.