With this change, POST requests that use multipart/form-data will use external-format-out for encoding, instead of :latin-1. Without this change, an attempt to use characters outside the :latin-1 range (e.g, U+2011, "n-dash") would cause an exception while building the request. Note: it would be possible to only do this for strings that have characters that are not standard-char.
From RFC 2616, section 4.4: 3.If a Content-Length header field (section 14.13) is present, its decimal value in OCTETs represents both the entity-length and the transfer-length. The Content-Length header field MUST NOT be sent if these two lengths are different (i.e., if a Transfer-Encoding header field is present). If a message is received with both a Transfer-Encoding header field and a Content-Length header field, the latter MUST be ignored.