content type is one of the fields which is almost always required for an application to function upon the sent data. it's not by RFC, but i noticed most servers and libraries won't handle requests missing this header properly (likely because they will have to guess media type and you will be lucky if it guesses the right one...)

from rfc: A sender that generates a message containing a payload body SHOULD generate a Content-Type header field in that message unless the intended media type of the enclosed representation is unknown to the sender. If a Content-Type header field is not present, the recipient MAY either assume a media type of "application/octet-stream" ([RFC2046], Section 4.5.1) or examine the data to determine its type.

here it says SHOULD, which is:

3. SHOULD This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.

https://tools.ietf.org/html/rfc2616

Ran into so many things that just don't respond atall when that header is there, that i made it practice just to include it in every http request i make, even if it's mainly just plaintext media type which you fill in... you can see that by rfc one would assume it's an octet-stream, so an xml would never be excpected if someone built to specifications.