add note on origins w/ default port to README.md (#163)

* Allow specifying redundant default port in origin

When `http://EXAMPLE:80` is an allowed origin, requests are not allowed from
`http://EXAMPLE`. Since port 80 is the default port for HTTP, browsers
will strip it and thus rack-cors never receives a request from `http://EXAMPLE`.

A similar problem is discussed here: request/request#515

* Remove default port from http(s) without URI#parse

* revert code changes

* add note on origins w/ default port to README.md

README.md

@@ -107,7 +107,9 @@ A Resource path can be specified as exact string match (`/path/to/file.txt`) or
 
 ### Origin Matching
 
-When specifying an origin, make sure that it does not have a trailing slash.
+* When specifying an origin, make sure that it does not have a trailing slash.
+
+* When specifying an HTTP origin that uses the scheme's default port (e.g. `http://example.test:80`), some clients may not strip the port which could result in unexpected blocked requests (additional context [here](https://github.com/request/request/pull/2904)).
 
 ### Testing Postman and/or cURL