De-duplicates cocaine command line path per run

Adds failing spec to command line dup check

[fixes #1670]
[fixes #2169]

lib/paperclip/helpers.rb

@@ -24,7 +24,8 @@ module Paperclip
     #
     def run(cmd, arguments = "", interpolation_values = {}, local_options = {})
       command_path = options[:command_path]
-      Cocaine::CommandLine.path = [Cocaine::CommandLine.path, command_path].flatten.compact.uniq
+      cocaine_path_array = Cocaine::CommandLine.path.try(:split, Cocaine::OS.path_separator)
+      Cocaine::CommandLine.path = [cocaine_path_array, command_path].flatten.compact.uniq
       if logging? && (options[:log_command] || local_options[:log_command])
         local_options = local_options.merge(:logger => logger)
       end

spec/paperclip/paperclip_spec.rb

@@ -29,7 +29,9 @@ describe Paperclip do
       Paperclip.options[:command_path] = "/opt/my_app/bin"
       Paperclip.run("convert", "stuff")
       Paperclip.run("convert", "more_stuff")
-      assert_equal 1, [Cocaine::CommandLine.path].flatten.size
+
+      cmd_path = Paperclip.options[:command_path]
+      assert_equal 1, Cocaine::CommandLine.path.scan(cmd_path).count
     end
   end