Http Message Versions Save

Immutability + URI interface

• null is no longer allowed (per the ML; see also php-fig/fig-standards#367).

• Refactors MessageInterface to only provide getters.
• MessageInterface now defines getBody() to require that it return a StreamableInterface instance.
• Removes Request and Response interfaces
• Provides server-side interfaces:
  • IncomingRequestInterface, which provides accessors for HTTP properties and environment-specific items ($_SERVER, $_GET, $_POST, $_FILES, $_COOKIE, etc), and support for mutable "attributes".
  • OutgoingResponseInterface, which provides both accessors and mutators for all HTTP properties.
• Provides client-side interfaces:
  • OutgoingRequestInterface, which provides accessors and mutators for all HTTP properties.
  • IncomingResponseInterface, which provides accessors for HTTP properties.
• StreamableInterface removes attach().

• Removed references to RFC 2616 (https://github.com/php-fig/fig-standards/pull/346)
• Added @throws annotations (https://github.com/php-fig/fig-standards/pull/345)
• Clarify MessageInterface::getHeaders() (https://github.com/php-fig/fig-standards/pull/347)
• Whitespace!!! (https://github.com/php-fig/fig-standards/pull/351)
• Clarify MessageInterface header methods (https://github.com/php-fig/fig-standards/pull/353)
• Clarify superglobal usage (https://github.com/php-fig/fig-standards/pull/354)
• Clarifications in language. (https://github.com/php-fig/fig-standards/pull/356)