ESP8266 returning 400 bad request
Take a look at the HTTP specification:

Request       = Request-Line              ; Section 5.1
                *(( general-header        ; Section 4.5
                 | request-header         ; Section 5.3
                 | entity-header ) CRLF)  ; Section 7.1
                CRLF
                [ message-body ]          ; Section 4.3

Request-Line  = Method SP Request-URI SP HTTP-Version CRLF


You don't send the CRLF (carriage return, line feed) at the end of your request line and at the end of the header at the same time of the request. Why the delay?

You also miss the obligatory host header.

Host = "Host" ":" host [ ":" port ] ; Section 3.2.2