filter out empty headers - resolves #53 (#65)

Fixes issues tpyo#157 & tpyo#171.

It turns out that ListBuckets does not work on non-amazon services due to a 403 SignatureDoesNotMatch error caused by the empty headers.

Co-authored-by: dcmexy <dcmexy@github.com>

S3.php

@@ -2004,7 +2004,7 @@ public static function __getSignatureV4($amzHeaders, $headers, $method, $uri, $p
 
 		// CanonicalHeaders
 		foreach ($headers as $k => $v)
-			$combinedHeaders[strtolower($k)] = trim($v);
+			if (!empty($v)) $combinedHeaders[strtolower($k)] = trim($v);
 		foreach ($amzHeaders as $k => $v) 
 			$combinedHeaders[strtolower($k)] = trim($v);
 		uksort($combinedHeaders, array('S3', '__sortMetaHeadersCmp'));