fix #1

[Nyholm]

Since Flysystem removes "any kind of funky unicode whitespace" we might run into a situation like this:

// Empty cache
$item = $pool->getItem('foo[funky unicode]bar");
$item->set('value');
$pool->save($item);

$pool->hasItem('foobar"); // True

This PR prevents that situation.

This will fix #1

[codecov-io]

Current coverage is 100.00%

Merging #3 into master will not affect coverage as of 881a6dc

@@            master      #3   diff @@
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  Files            1       1       
  Stmts           25      27     +2
  Branches         0       0       
  Methods          6       6       
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+ Hit             25      27     +2
  Partial          0       0       
  Missed           0       0       

Review entire Coverage Diff as of 881a6dc

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