Prevent duplicate no fills (#1086)

* Merge branch 'master' of C:\Projects\PHPOffice\PHPSpreadsheet\develop with conflicts.

* Ignore colours for hash generation when fill style in NONE

* Update changelog

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 
 ### Fixed
 
+- Eliminate duplicate fill none style entries [Issue #1066](https://github.com/PHPOffice/PhpSpreadsheet/issues/1066) 
 - Fix number format masks containing literal (non-decimal point) dots [Issue #1079](https://github.com/PHPOffice/PhpSpreadsheet/issues/1079) 
 - Fix number format masks containing named colours that were being misinterpreted as date formats; and add support for masks that fully replace the value with a full text string [Issue #1009](https://github.com/PHPOffice/PhpSpreadsheet/issues/1009) 
 - Stricter-typed comparison testing in COUNTIF() and COUNTIFS() evaluation [Issue #1046](https://github.com/PHPOffice/PhpSpreadsheet/issues/1046)

src/PhpSpreadsheet/Style/Fill.php

@@ -313,12 +313,13 @@ class Fill extends Supervisor
         if ($this->isSupervisor) {
             return $this->getSharedComponent()->getHashCode();
         }
-
+        // Note that we don't care about colours for fill type NONE, but could have duplicate NONEs with
+        //  different hashes if we don't explicitly prevent this
         return md5(
             $this->getFillType() .
             $this->getRotation() .
-            $this->getStartColor()->getHashCode() .
-            $this->getEndColor()->getHashCode() .
+            ($this->getFillType() !== self::FILL_NONE ? $this->getStartColor()->getHashCode() : '') .
+            ($this->getFillType() !== self::FILL_NONE ? $this->getEndColor()->getHashCode() : '') .
             __CLASS__
         );
     }