Ensure that an empty array cannot be provided as a set of headers

PSR-7 requires a string or array of strings; if an array is provided, it
cannot be empty.

Author: weierophinney

src/MessageTrait.php

@@ -385,6 +385,13 @@ private function filterHeaderValue($values)
             $values = [$values];
         }
 
+        if ([] === $values) {
+            throw new InvalidArgumentException(
+                'Invalid header value: must be a string or array of strings; '
+                . 'cannot be an empty array'
+            );
+        }
+
         return array_map(function ($value) {
             HeaderSecurity::assertValid($value);