fix missing \r\n after Content-Disposition in iOS

[5uper]

This is related on feat(http): support for FormData requests #6708

When comparing FormData with the type string and base64File we see that in the case string a second \r\n is missing after content disposition in iOS. This leads to the fact that we receive a 400 Bad Request from the server with a request from iOS.

In HTTP version 1.x, header fields are transmitted after the request line (in case of a request HTTP message) or the response line (in case of a response HTTP message), which is the first line of a message. Header fields are colon-separated key-value pairs in clear-text string format, terminated by a carriage return (CR) and line feed (LF) character sequence. The end of the header section is indicated by an empty field line, resulting in the transmission of two consecutive CR-LF pairs.
https://en.wikipedia.org/wiki/List_of_HTTP_header_fields