charset on Messaage is now an Option
Change default xml deocoding charset to US-ASCII

This is per spec
https://www.ietf.org/rfc/rfc3023.txt section 3.1

"Conformant with [RFC2046], if a text/xml entity is received with
 the charset parameter omitted, MIME processors and XML processors
 MUST use the default charset value of "us-ascii"[ASCII]."
Merge pull request #155 from http4s/topic/OptionCharset

Topic/option charset