Tons of tests for Dancer2::Core::Request:

While these aren't all the tests, it took a while to write.

It covers a fair portion of the ::Request object, and it makes
t/request_params.t obsolete.

The following is covered:

* Methods:
    * var
    * address
    * protocol
    * remote_host
    * forwarded_for_address
    * forwarded_host
    * forwarded_protocol
    * port
    * request_uri
    * user
    * script_name
    * to_string
    * path
    * path_info
    * is_head
    * is_post
    * is_get
    * is_put
    * is_delete
    * is_patch

* Attribute:
    * env
    * id
    * accept
    * accept_charset
    * accept_encoding
    * accept_language
    * connection
    * keep_alive
    * referer
    * user_agent
    * x_requested_with
    * vars
    * path
    * path_info
    * method
    * serializer
    * _query_params
    * _body_params
    * _mixed_params

* Attribute aliases:
    * agent -> user_agent
    * remote_address -> address

Things I'd like to check more thoroughly:
* path/path_info
* forwarded_for_address
* forwarded_protocol
* forwarded_host
* host
* is_behind_proxy
* scheme
* Uploads
* Cookies
* Headers
fix test
Require env hash, no default:

There should be no default to the request object. It should be
constructed from an environment hash.
Provide forwarded_host and forwarded_protocol:

We document they exist, but they didn't until now.
Cleanups:

All PSGI arguments are in lowercase (psgi.url_scheme, psgi.input).
Document the request ID:

Why not?
remove obsolete test
add X_FORWARDED_PROTO too
reflect changes
Merge branch 'tests/request'