Add option to not require content type header

[mcarlin]

Problem

There are some clients that don't send a content-type header but send an XML body. It'd be nice to be able to support these clients by disabling the content-type check.

Possible Solution

Add a bool field to XmlConfig to allow disabling of content type checking. This would default to the existing behavior but would allow consumers of this crate to turn it off if necessary.

If you're open to PRs, I can tackle adding this feature.