Fix forming signature with multiple space (#981)

Ref https://github.com/aws/aws-sdk-php/blob/master/src/Signature/SignatureV4.php#L301

Problem example:

```php
$s3->copyObject([
  'Bucket' => 'target',
  'Key' => '1.txt',
  'CopySource' => 'source/1  2.txt', // double space
]);
```

Author: vlastv

src/Signer/SignerV4.php

@@ -273,7 +273,7 @@ private function buildCanonicalHeaders(Request $request, bool $isPresign): array
                 continue;
             }
 
-            $canonicalHeaders[$key] = "$key:$value";
+            $canonicalHeaders[$key] = $key . ':' . preg_replace('/\s+/', ' ', $value);
         }
         ksort($canonicalHeaders);