Feature / Fix: Pass custom request headers when following redirects (#91 by @seregazhuk and #96 by @clue)

Support legacy PHP 5.3 through PHP 7.2 and HHVM (#95 by @clue)

Improve documentation (#87 by @holtkamp and #93 by @seregazhuk)

Improve test suite by adding forward compatibility with PHPUnit 5, PHPUnit 6 and PHPUnit 7 and explicitly test HTTP/1.1 protocol version. (#86 by @carusogabriel and #94 and #97 by @clue)