Source

File: wp-includes/rest-api/class-wp-rest-request.php

public function add_header( $key, $value ) {
		$key   = $this->canonicalize_header_name( $key );
		$value = (array) $value;

		if ( ! isset( $this->headers[ $key ] ) ) {
			$this->headers[ $key ] = array();
		}

		$this->headers[ $key ] = array_merge( $this->headers[ $key ], $value );
	}