Description:
------------
When I run the test script with latest mailparse(master branch; bc51125921ae1287e3b4705bec1dfd8859d16213 ) and PHP 7.1.2(or PHP7.0.30),
memory is not freed.
When I run it with debug mode, any memory leak is not reported.

But with PHP 5.6.30, memory is freed.

Test script:
---------------
<?php

echo('Before: ' . memory_get_usage() . PHP_EOL);
for ($i = 0; $i < 10000; $i++) {
    if ($i % 1000 == 0)
        echo(' .. ' . memory_get_usage() . PHP_EOL);
    new MimeMessage('file', 'tests/testdata/m0001.txt');
}
echo('After: ' . memory_get_usage() . PHP_EOL);
gc_collect_cycles();
echo('After(gc_collect_cycles): ' . memory_get_usage() . PHP_EOL);

Expected result:
----------------
# This is a result of PHP 5.6.30 and mailparse 2.1.6

Before: 225304
 .. 225488
 .. 225552
 .. 225552
 .. 225552
 .. 225552
 .. 225552
 .. 225552
 .. 225552
 .. 225552
 .. 225552
After: 225552
After(gc_collect_cycles): 225552

Actual result:
--------------
# This is a result of PHP 7.1.2 and latest mailparse 

Before: 350416
 .. 350448
 .. 760368
 .. 1166512
 .. 1834800
 .. 1978800
 .. 3171400
 .. 3315400
 .. 3459400
 .. 3603400
 .. 5844552
After: 5988552
After(gc_collect_cycles): 5988552