src/HttpHeaders/AddHttpHeadersMiddleware.php
public function addHeader(string $header, $value):void
	{
	    if (is_iterable($value) || is_array($value)) {
	        $iterable = $value;
	        $value = [];
	        foreach ($iterable as $key => $entry) {
                $value[$key] = (string)$entry;
            }
        }
        else {
	        $value = (string)$value;
        }

		$this->headers[] = [$header, $value];
	}
public function addHeader(string $header, $value):void
	{
	    if (is_iterable($value) || is_array($value)) {
	        $iterable = $value;
	        $value = [];
	        foreach ($iterable as $key => $entry) {
                $value[$key] = (string)$entry;
            }
        }
        else {
	        $value = (string)$value;
        }

		$this->headers[] = [$header, $value];
	}
Adds one header.

@param string $header
@param string|array|iterable $value