Description:
------------
Running:
Windows 10
WAMP 3.0.8
PHP 7.0.10
Apache 2.4.23
After compression of Phar file, method Phar::isCompressed() always returns FALSE.

Searched and scoured all previous bug reports. This is a new bug.

Test script:
---------------
    $phar1 = new Phar('myphar.zip.phar');
    var_dump($phar1->isCompressed());
    $phar2 = new Phar('myuncompressed.tar.phar');
	$phar2['x.txt']='my x';
    var_dump($phar2->isCompressed());
    $phar2->compress(Phar::GZ);
    var_dump($phar2->isCompressed() == Phar::GZ);

Expected result:
----------------
bool(false)
bool(false)
bool(true)

Actual result:
--------------
bool(false)
bool(false)
bool(false)