Fixed wrong Content-Length header when sending file using Response::sendContentAsFile().

Makes it possible for classes that extend yii\web\ErrorHandler to provide their own informational URLs for types/classes/methods.

- Made yii\web\Request::getQueryParam() and getBodyParam()'s doc blocks more consistent with each other
- getQueryParam() was incorrectly stating that if $name was omitted, all of $_GET would be returned (but $name is required)

Fixes #6398: Added support for specifying dependent component in terms of a configuration array for classes such as `DbCache`

`Content-Transfer-Encoding` is not a valid HTTP header (cf [RFC 7230, section 3.3.1](http://tools.ietf.org/html/rfc7230#section-3.3.1), [RFC 7231, section 8.3](http://tools.ietf.org/html/rfc7231#section-8.3), [IANA Message Header Registry](http://www.iana.org/assignments/message-headers/message-headers.xhtml)). The correct HTTP pendant would be `Transfer-Encoding: identity`, which were entirely bogus.

fixes #6717

introduced in https://github.com/yiisoft/yii2/commit/bf206efc66bc47b693271359e433679c1618189b

Fixes #6427: In case of invalid route web application now throws exception with "Page not found" instead of "Invalid Route"

Fixes #4395: Added `$checkAjax` parameter to `yii\web\Response::redirect()` to support default redirection behavior for AJAX/PJAX requests

Fixes #5480: Added defensive code to `yii\web\User::getIdentity()` to avoid potential infinite recursion

Fixes #4889: Application was getting into redirect loop when user wasn't allowed accessing login page. Now shows 403

close #5762

Fixes #5587: `json_encode` is now used with `JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE` where it makes sense, also it is now default for `Json::encode()`

If you don't want to install command-line utils globally, you have to specify it's paths.

    public $commands = [
        'styl' => ['css', '@app/node_modules/bin/stylus < {from} > {to}'],
    ];

close #5130