Rename parameters to fix nanodicom under php7

[jacobalberty]

Without this patch running nanodicom under php7 throws a "Redefinition of parameter" compile time error.

[yeoupooh]

Same problem in using php 7.1.

[D4V3M0NK]

core.php:1082 has $arg2 defined twice in _dummy(). It just needs a slight update to remove the second:

from 1082: protected function _dummy($arg1 = NULL, $arg2 = NULL, $arg2 = NULL, ... ______________________________________________[keep]________^remove_this^

1082: protected function _dummy($arg1 = NULL, $arg2 = NULL, $arg3 = NULL, ...

[uiosun]

the PR(https://github.com/nanodocumet/Nanodicom/pull/20) fix it?