Add support for preparing a WebResource before serving for bits which render templates etc and need to be rendered before bytes and/or length can be known.
check state sanity for prepared resource
content-type is nullable handle null case, add assert to make IDEA shut-up
always set content-type, use application/octet-stream for unknown.
add constant for unknown content-type
update docs
Merge pull request #289 from sonatype/preparable-web-resources

REVIEW: Add support for preparing a WebResource before serving