Error when content-type is incorrect

[wgahnagl]

Hey! I've hit this error when receiving improperly formatted headers from remote servers. I know this is definitely supposed to work as written, and it is the server's fault for being improperly formatted, but it still makes it impossible to use this library's responses, despite the actual body of the response being valid xml.

Do you think there would be any value in creating a way to deliberately override or skip this function's content-type check?

https://github.com/jonas-schievink/xml-rpc-rs/blob/2dde66230a1f84129cf662cdd2a55e771ccb61ee/src/transport.rs#L119