fix bool(CacheInfo) on PY3
decode headers from CGI
encode WMS POST data (SLD)
combine featureinfo as bytes
fix loading of encoded sld_body data
simplify xml header in tms templates

remove encoding (utf8 by default anyways) to allow lxml to
parse it as string (instead of byte) in tests
update test cgi script for python 3
fix tests for python 3

- send dummy data as bytes and compare with bytes
- fix imports
- use bbox_aware_query_comparator to fix false mockserv errors
encode data before hashing
remove unused function
fix doctests
skip mapproxy-util system tests on PY3 for now