changeed it back 'consist' => the 'must' should be applied to all par…

…ts of the sentence

src/Header/GenericHeader.php

@@ -84,7 +84,7 @@ public function setFieldName($fieldName)
         // Validate what we have
         if (!preg_match('/^[a-z][a-z0-9-]*$/i', $fieldName)) {
             throw new Exception\InvalidArgumentException(
-                'Header name must start with a letter, and consists of only letters, numbers and dashes.'
+                'Header name must start with a letter, and consist of only letters, numbers and dashes.'
             );
         }