Mojo::Message doesn't parse messages without a content-length header.

I was abusing Mojo::Message in [XDeadly::Post](https://github.com/afresh1/xdeadly/blob/master/lib/XDeadly/Post.pm) and found it wasn't doing what I wanted.  I'm unsure my fix is correct but it seems to at least work.

It could also be that I'm just using it wrong.