Updates from Review-2

This tag contains all updates since entering the second review of PSR-7. These include:

• Clarifications to RequestInterface::withUri() specifying that changes MUST also update the Host header unless the $preserverHost flag is true.
• Removal of the various getHeader*() method overrides in RequestInterface, as they are irrelevant with the above change.
• Added verbiage to the RequestInterface class-level docblock indicating that constructors SHOULD attempt to set the Host header in order to ensure the message is valid on initialization.
• RequestInterface::getHeaderLine() MUST return a string (null was allowed previously, which leads to ambiguity of return value).
• ResponseInterface::getReasonPhrase() MUST return a string(null was allowed previously, which leads to ambiguity of return value).
• ResponseInterface::withStatus() now sets the default value of the $reasonPhrase parameter to an empty string (to correspond to the above change).
• UploadedFileInterface::move() was renamed to moveTo(), and the argument renamed to $targetPath; this clarifies intent for the method.
• UriInterface::withPath() has clarifications in the docblock to indicate the four possible path types (empty, root path, relative path, domain-relative — or absolute — path).