Object injection

SECURITY Fix potential object injection vulnerability. CVE-2018-19296. Reported by Sehun Oh of cyberone.kr. Added Tagalog translation, thanks to @StoneArtz Added Malagache translation, thanks to @Hackinet Updated Serbian translation, fixed incorrect language code, thanks to @mmilanovic4 Updated Arabic translations (@MicroDroid) Updated Hungarian translations Updated Dutch translations Updated Slovenian translation (@filips123) Updated Slovak translation (@pcmanik) Updated Italian translation (@sabas) Updated Norwegian translation (@aleskr) Updated Indonesian translation (@mylastof) Add constants for common values, such as text/html and quoted-printable, and use them Added support for copied headers in DKIM, helping with debugging, and an option to add extra headers to the DKIM signature. See DKIM_sign example for how to use them. Thanks to @gwi-mmuths. Add Campaign Monitor transaction ID pattern matcher Remove deprecated constant and ini values causing warnings in PHP 7.3, added PHP 7.3 build to Travis config. Expanded test coverage