Explicitly require "uri" for 1.9.2-preview3+

* The URI namespace is not automatically loaded in 1.9.2-preview3 and above
* Moved constant declaration to the top of the class
* Froze constant (in general constants should be frozen)
* Reordered require statements for core libs to be alphabetical

[#1304 state:resolved]

lib/extlib/hash.rb

@@ -1,12 +1,15 @@
-require "time"
 require 'bigdecimal'
+require 'time'
+require 'uri'
 
 require 'extlib/object'
 require 'extlib/mash'
 require 'extlib/class'
 require 'extlib/string'
 
 class Hash
+  URI_ENCODE_PATTERN = Regexp.new("[^#{URI::PATTERN::UNRESERVED}]").freeze
+
   class << self
     # Converts valid XML into a Ruby Hash structure.
     #
@@ -126,7 +129,6 @@ def to_params
   # @return [String] This key value pair as a param
   #
   # @api public
-  URI_ENCODE_PATTERN = Regexp.new("[^#{URI::PATTERN::UNRESERVED}]")
   def normalize_param(key, value)
     param = ''
     stack = []