fix cs develop
.php_cs rule reverted
revert join_function from .php_cs
Merge pull request #7081 from samsonasik/dev-cs-try-fix

try fix travis cs develop again
implement @todo for setters/getters at Zend\Mime\Part
setContent($content) need is_resource check
use $this->testText as defined in setUp
added InvalidArgumentException test and throws
added missing @throws InvalidArgumentException
added Mime\Exception\InvalidArgumentException instead
fixes cs
fixes @throw docblock