Source

File: wp-includes/class-wp-http-requests-response.php

public function get_headers() {
		// Ensure headers remain case-insensitive.
		$converted = new Requests_Utility_CaseInsensitiveDictionary();

		foreach ( $this->response->headers->getAll() as $key => $value ) {
			if ( count( $value ) === 1 ) {
				$converted[ $key ] = $value[0];
			} else {
				$converted[ $key ] = $value;
			}
		}

		return $converted;
	}