Minor tweaks to use_in_file_templates! auto loading

1. Don't add the sinatra lib dir to $: on each reload.
2. Don't remove sinatra.rb from $LOADED_FEATURES unless we're
   reloading.

lib/sinatra.rb

@@ -1,4 +1,6 @@
-$LOAD_PATH.unshift File.dirname(__FILE__)
+libdir = File.dirname(__FILE__)
+$LOAD_PATH.unshift(libdir) unless $LOAD_PATH.include?(libdir)
+
 require 'sinatra/base'
 require 'sinatra/main'
 require 'sinatra/compat'

lib/sinatra/base.rb

@@ -796,7 +796,6 @@ module Sinatra
     end
 
     def self.call(env)
-      $LOADED_FEATURES.delete("sinatra.rb")
       reload! if reload?
       super
     end
@@ -804,6 +803,7 @@ module Sinatra
     def self.reload!
       @reloading = true
       superclass.send :inherited, self
+      $LOADED_FEATURES.delete("sinatra.rb")
       ::Kernel.load app_file
       @reloading = false
     end