Onf of the sides can be nil and it will raise a Conversion error

The previous regex was allowing `_` in the URI scheme, which is not
allowed by RFC 3986. This change brings the regex in line with the RFC.

In some instances, `assert_redirected_to` assertion was returning an
incorrect and misleading failure message when the assertion failed.
This was due to a disconnect in how the assertion computes the redirect
string for the failure message and how `redirect_to` computes the
string that is actually used for redirection.

I made the `_compute_redirect_to_loaction` method used by `redirect_to`
public and call that from the method `assert_redirect_to` uses to
calculate the URL.

The reveals a new test failure due to the regex used by
`_compute_redirect_to_location` allow `_` in the URL scheme.

Previously it was raising a NilException

This reverts commit ab5cd54b, reversing
changes made to cdc10c89.

Reason: This way of defining root path is still supported. See
https://github.com/rails/rails/blob/d262773ab7f0aae5de2d354ac2eca168e95b653d/actionpack/test/controller/routing_test.rb#L450-457

Reference:
Bloody mess internals
http://gusiev.com/slides/rails_contribution/static/#40

The documentation in this section is referring to a profile,
so the resource that's created should probably also be a
profile of some sort.

The docs refference a blacklist, but really what's being described
is a whitelist. Anything that matches the constraint gets through to
the path.

closes #12149

The methods:

 * #render_to_body
 * #render_to_string
 * #_normalize_render

Haven't had anything specyfic to ActionView. This was common code which should belong to AbstractController

This reverts commit 7de994fa.

This piece of documentation is out of date.
The use of match without any via option is prevented, now
the HTTP verbs have to be explicitly set. If they're not set then
the error message in normalize_conditions! (around line 186) is
shown.

Make it oneliner

We're not dealing with templates here

[ci skip]