yaws_api: Fix matching of Content-Type in parse_post/1

"application/x-www-form-urlencoded" can be followed by attributes, such
as "; charset=UTF-8".
Merge pull request #180 from yakaz/fix-contenttype-matching-in-yaws_api

yaws_api: Fix matching of Content-Type in `parse_post/1`
Use iolist_to_binary/iolist_size instead of custom functions

And avoid some useless conversions
Merge remote-tracking branch 'upstream/master'