Update PSR-2-coding-style-guide.md

Fix typos
Moving design decisions into the meta document
Updating HTTP message proposal based on feedback

- https://groups.google.com/forum/?fromgroups#!topic/php-fig/BjuMHOzTUJs
- Added link to RFC 2616
- Added getHeaderAsArray()
- Added more information about how URLs are absolute URLs
- Returning null if a stream size cannot be determined
- Specifying the protocol version format
Merge pull request #288 from mtdowling/http-message-updates

Http message updates based on feedback
Cleanup on PSR-4.
Switch to HttpMessage
Merge pull request #287 from Adel-E/patch-1

Update PSR-2-coding-style-guide.md
Fixed #290: RFC2616 -> RFC7230
Removing fluent interface from HTTP Message interface
Removing fluent interface from HTTP Request interface
Explicitly adding `@return void` to avoid any kind of ambiguity