Update to latest PSR-7 version

[weierophinney]

Syncs all interface definitions with the latest revisions on php-fig/fig-standards.

This PR contains changes from the following as-yet unmerged pull-requests, and should therefore be merged after them:

• Clarification of withPath fig-standards#543
• Changed "case-insensitive" to "case-sensitive" in RequestInterface::with... fig-standards#544

[weierophinney]

Once merged, please tag 0.11.0, with the following:

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).

[weierophinney]

Pinging @simensen and/or @pmjones — once the other PRs I've approved are merged, merge this one and tag, please.

[pmjones]

Tagged.