Summary:
RFC 6454 section 7 defines the Origin: header syntax, and it's a
scheme, host, and optional port, not a URL.
Section 6 defines serialization of the header, including omission of the
port.
Therefore, we need to omit the trailing slash in all cases, and omit
the port if it matches the default port for the protocol.
Closes https://github.com/facebook/react-native/pull/7920
Differential Revision: D3387619
fbshipit-source-id: 552756e63ad41463af357a5073fae56c96e58958
03512fb721