Now preserving case of custom doctypes

The doctype is case sensitive in XML files but custom doctypes was mistakenly
made lower case previous to this commit.

Added test case and also updated type in README where doctypes where described.


Currently:

> doctype cross-domain-policy PUBLIC "http://www.macromedia.com/xml/dtds/cross-domain-policy.dtd"

Generates (notice that PUBLIC is made lower case):

> <!DOCTYPE cross-domain-policy system "http://www.macromedia.com/xml/dtds/cross-domain-policy.dtd">

This makes validating XML parsers choke with errors such as "DOCTYPE improperly terminated".

With the pull request the case is preserved and everyone is happy.