Making JSON-RPC accept more content types

According to the RFC for JSON-RPC over HTTP at
http://www.jsonrpc.org/historical/json-rpc-over-http.html
"Content-Type SHOULD be 'application/json-rpc' but MAY be
'application/json' or 'application/jsonrequest'"

This patch fixes HTTPJsonRpcHandler to allow for any of those three
content types when accepting JSON-RPC HTTP requests.

This also fixes compatibility with the popular Python library jsonrpclib
which uses the application/json-rpc content type.
[codegenerator] Helper.groovy: fix typo
[codegenerator] Helper.groovy: add support for class variables
[codegenerator] python: extend IN typemap to support int, double and float
DVDAudioCodecFFmpeg: Grow the resampling buffer as needed instead of allocating it an arbitrary big size.
DVDAudioCodecFFmpeg: Remove write only assignment.
DllAvFilter: Always include libavfilter/buffersrc.h for av_buffersrc_add_frame.
[codegenerator] python: extend swig template to support class variables
[legacy] hide C++ only public class variables from swig
python: don't consider scripts which threw an exception as successful
fixed: Crash when scanning music without any scraper set
changed: It's nicer to close the music db on failure
fixed: Music album info gets stuck on failure (fixes #14435)
fixed, update of rss feed URLS
changed: Make more obvious we're passing a CFileItem
fixed: SQL query was wrong
fixed: Formatting (cosmetics)
fixed: artist was being added as genre to music album table
fixed: Locally found music album art was not stored in the db ((partially?) fixes #14345)
Merge pull request #2903 from davilla/change-rss-urls

fixed, update of rss feed URLS