remove deprecated Content-Transfer-Encoding from multipart post headers

[mtrna]

motivation:

This PR fixes a problem I stumbled upon: rest-assured requests are rejected by receiver because of invalid header in multipart post.

According to https://datatracker.ietf.org/doc/html/rfc7578#section-4.7 senders SHOULD NOT generate any parts with a Content-Transfer-Encoding header field.

sources:

• SO issue related directly to rest-assured: https://stackoverflow.com/questions/72868970/javarestassured-remove-content-transfer-encoding-binary-from-multipart-request
• another probable manifestation of the root cause coming from (possibly incorrect usage of) the underlying apache library elsewhere: https://stackoverflow.com/questions/33023370/setting-the-content-transfer-encoding-of-upload-file-in-multi-part-http-request

compare/contrast:

cURL 7.68.0 for -F attachments=@attachment.txt sends e.g.:

Content-Disposition: form-data; name="attachments"; filename="attachment.txt"
Content-Type: text/plain

test

rest-assured 5.4.0 for .multiPart("attachments", new File("attachment.txt"), "text/plain") without fix sends e.g.:

Content-Disposition: form-data; name="attachments"; filename="attachment.txt"
Content-Type: text/plain
Content-Transfer-Encoding: binary

test

rest-assured 5.4.0 with this fix sends e.g.:

Content-Disposition: form-data; name="attachments"; filename="attachment.txt"
Content-Type: text/plain

test

[mtrna]

@johanhaleby any suggestions how to steer the fix to the next release? :)