src/AbstractTransport.php
protected function normalizeHeaderName($keyName)
    {
        if (!is_string($keyName)) {
            return null;
        }

        return ucwords(trim(strtolower($keyName)), '-') ?: null;
    }
protected function normalizeHeaderName($keyName)
    {
        if (!is_string($keyName)) {
            return null;
        }

        return ucwords(trim(strtolower($keyName)), '-') ?: null;
    }
Normalize Real Header Name

@param string $keyName Header Name key, The key will be convert into
First Character after `-`(dash) into uppercase
And space will be replace as `dash`
@return null|string