Pass extra rubocop options in the middle instead, and fix the Vader test so it can be run multiple times in Vim

autoload/ale/fixers/rubocop.vim

@@ -4,11 +4,12 @@ function! ale#fixers#rubocop#GetCommand(buffer) abort
     \   ? ' exec rubocop'
     \   : ''
     let l:config = ale#path#FindNearestFile(a:buffer, '.rubocop.yml')
+    let l:options = ale#Var(a:buffer, 'ruby_rubocop_options')
 
     return ale#Escape(l:executable) . l:exec_args
     \   . (!empty(l:config) ? ' --config ' . ale#Escape(l:config) : '')
-    \   . ' --auto-correct %t '
-    \   . ale#Var(a:buffer, 'ruby_rubocop_options')
+    \   . (!empty(l:options) ? ' ' . l:options : '')
+    \   . ' --auto-correct %t'
 
 endfunction