update Hash documentation with 1.9 syntax [ci skip]

Author: Francesco Rodriguez

activesupport/lib/active_support/core_ext/hash/keys.rb

@@ -1,8 +1,10 @@
 class Hash
   # Return a new hash with all keys converted using the block operation.
   #
-  #  { :name => 'Rob', :years => '28' }.transform_keys{ |key| key.to_s.upcase }
-  #  # => { "NAME" => "Rob", "YEARS" => "28" }
+  #  hash = { name: 'Rob', age: '28' }
+  #
+  #  hash.transform_keys{ |key| key.to_s.upcase }
+  #  # => { "NAME" => "Rob", "AGE" => "28" }
   def transform_keys
     result = {}
     keys.each do |key|
@@ -22,8 +24,10 @@ def transform_keys!
 
   # Return a new hash with all keys converted to strings.
   #
-  #   { :name => 'Rob', :years => '28' }.stringify_keys
-  #   #=> { "name" => "Rob", "years" => "28" }
+  #   hash = { name: 'Rob', age: '28' }
+  #
+  #   hash.stringify_keys
+  #   #=> { "name" => "Rob", "age" => "28" }
   def stringify_keys
     transform_keys{ |key| key.to_s }
   end
@@ -37,8 +41,10 @@ def stringify_keys!
   # Return a new hash with all keys converted to symbols, as long as
   # they respond to +to_sym+.
   #
-  #   { 'name' => 'Rob', 'years' => '28' }.symbolize_keys
-  #   #=> { :name => "Rob", :years => "28" }
+  #   hash = { 'name' => 'Rob', 'age' => '28' }
+  #
+  #   hash.symbolize_keys
+  #   #=> { name: "Rob", age: "28" }
   def symbolize_keys
     transform_keys{ |key| key.to_sym rescue key }
   end
@@ -69,8 +75,10 @@ def assert_valid_keys(*valid_keys)
   # This includes the keys from the root hash and from all
   # nested hashes.
   #
-  #  { :person => { :name => 'Rob', :years => '28' } }.deep_transform_keys{ |key| key.to_s.upcase }
-  #  # => { "PERSON" => { "NAME" => "Rob", "YEARS" => "28" } }
+  #  hash = { person: { name: 'Rob', age: '28' } }
+  #
+  #  hash.deep_transform_keys{ |key| key.to_s.upcase }
+  #  # => { "PERSON" => { "NAME" => "Rob", "AGE" => "28" } }
   def deep_transform_keys(&block)
     result = {}
     each do |key, value|