Do not add leading slash to default admin path

Since 871e77e we allow to set a custom
admin path (ie. `/backend` instead of `/admin`). The default is set to
`/admin` and since the admin dashboard redirect rule from `/admin` to
`/admin/dashboard` uses this value the redirect destination is a root
path.

This causes no problems, unless you mount Alchemy at some other path
(`/cms` instead of just `/`). Because the redirect rule has a leading
slash the Rails router does not take the mount path into account and
redirects to `/admin/dashboard` instead of `/cms/admin/dashboard`.

Closes AlchemyCMS#1244

README.md

@@ -170,7 +170,7 @@ For example, these settings:
 ```ruby
 # config/initializers/alchemy.rb
 
-Alchemy.admin_path = '/backend'
+Alchemy.admin_path = 'backend'
 Alchemy.admin_constraints = {subdomain: 'hidden'}
 ```
 

lib/alchemy/paths.rb

@@ -4,7 +4,7 @@
 #
 # Alchemy has some defaults for admin path and admin constraints:
 #
-# +Alchemy.admin_path defaults to +'/admin'+
+# +Alchemy.admin_path defaults to +'admin'+
 # +Alchemy.admin_constraints defaults to +{}+
 #
 # Anyway, you can tell Alchemy about your routing configuration:
@@ -17,18 +17,18 @@
 #
 # == Example
 #
-# If you do not wish to use the default admin interface routing ('example.com/admin/')
-# and prefer e.g. 'hidden.example.com/backend/', those are the settings you need:
+# If you do not wish to use the default admin interface routing ('example.com/admin')
+# and prefer e.g. 'hidden.example.com/backend', those are the settings you need:
 #
 #     # config/initializers/alchemy.rb
-#     Alchemy.admin_path = '/backend'
+#     Alchemy.admin_path = 'backend'
 #     Alchemy.admin_constraints = {subdomain: 'hidden'}
 #
 module Alchemy
   mattr_accessor :admin_path, :admin_constraints
 
   # Defaults
   #
-  @@admin_path = '/admin'
+  @@admin_path = 'admin'
   @@admin_constraints = {}
 end