Fix bug where validating currency returns true after setting to null

Previously, if you set the currency field to null and then called
`validate` on the currency field, the validation would pass because
`strtoupper(null) === ''`, but `validate` looks for null values.

Now `strtoupper` is not called when the currency field is set to
null, so validation will fail.

Author: nickyr

src/Omnipay/Common/Message/AbstractRequest.php

@@ -399,7 +399,10 @@ public function getCurrency()
      */
     public function setCurrency($value)
     {
-        return $this->setParameter('currency', strtoupper($value));
+        if ($value !== null) {
+            $value = strtoupper($value);
+        }
+        return $this->setParameter('currency', $value);
     }
 
     /**

tests/Omnipay/Common/Message/AbstractRequestTest.php

@@ -284,6 +284,12 @@ public function testCurrencyLowercase()
         $this->assertSame('USD', $this->request->getCurrency());
     }
 
+    public function testCurrencyNull()
+    {
+        $this->assertSame($this->request, $this->request->setCurrency(null));
+        $this->assertNull($this->request->getCurrency());
+    }
+
     public function testCurrencyNumeric()
     {
         $this->assertSame($this->request, $this->request->setCurrency('USD'));