Fix regex used to find URI schemes in redirect_to

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.

actionpack/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Fix regex used to detect URI schemes in `redirect_to` to be consistent with
+    RFC 3986.
+
+    *Derek Prior*
+
 *   Fix incorrect `assert_redirected_to` failure message for protocol-relative
     URLs.
 

actionpack/lib/action_controller/metal/redirecting.rb

@@ -78,7 +78,7 @@ def _compute_redirect_to_location(options) #:nodoc:
       # characters; and is terminated by a colon (":").
       # See http://tools.ietf.org/html/rfc3986#section-3.1
       # The protocol relative scheme starts with a double slash "//".
-      when %r{\A(\w[\w+.-]*:|//).*}
+      when /\A([a-z][a-z\d\-+\.]*:|\/\/).*/i
         options
       when String
         request.protocol + request.host_with_port + options