PION-1311: Moved xml_encode() method from Cloudmeter Core library into
Pion's algorithm lib.

Using xml_encode() in FileService and http_server to prevent XSS attacks
PION-1312: patch to avoid access to uninitialized memory in http_parser.cpp

I believe this is related to and may have been at least mostly alleviated by
the earlier fix made for this: https://github.com/cloudmeter/pion/issues/18

This just includes a unit test + updates to make form content parsing safer
+ removes unnecessary extra work when in "headers only" parsing mode
PION-1313: parser enters infinite loop when payload_handler is defined

From paolovolpi @ GitHub https://github.com/cloudmeter/pion/issues/21
Merge remote-tracking branch 'upstream/develop' into develop

Conflicts:
	src/http_parser.cpp