Revert "Use flat_map { } instead of map {}.flatten"

This reverts commit abf8de85519141496a6773310964ec03f6106f3f.
We should take a deeper look to those cases flat_map doesn't do deep
flattening.

irb(main):002:0> [[[1,3], [1,2]]].map{|i| i}.flatten
=> [1, 3, 1, 2]
irb(main):003:0> [[[1,3], [1,2]]].flat_map{|i| i}
=> [[1, 3], [1, 2]]

1 files changed, 3 insertions, 3 deletions

diff --git a/guides/source/active_support_core_extensions.md b/guides/source/active_support_core_extensions.md
index 1930cbe22c..2a84242b9c 100644
--- a/guides/source/active_support_core_extensions.md
+++ b/guides/source/active_support_core_extensions.md
@@ -517,7 +517,7 @@ ActionController::TestCase.class_eval do
 
   # now redefine process and delegate to original_process
   def process(action, params=nil, session=nil, flash=nil, http_method='GET')
-    params = Hash[*params.flat_map {|k, v| [k, v.to_s]}]
+    params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
     original_process(action, params, session, flash, http_method)
   end
 end
@@ -530,7 +530,7 @@ That technique has a risk, it could be the case that `:original_process` was tak
 ```ruby
 ActionController::TestCase.class_eval do
   def process_with_stringified_params(...)
-    params = Hash[*params.flat_map {|k, v| [k, v.to_s]}]
+    params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
     process_without_stringified_params(action, params, session, flash, http_method)
   end
   alias_method :process_without_stringified_params, :process
@@ -543,7 +543,7 @@ The method `alias_method_chain` provides a shortcut for that pattern:
 ```ruby
 ActionController::TestCase.class_eval do
   def process_with_stringified_params(...)
-    params = Hash[*params.flat_map {|k, v| [k, v.to_s]}]
+    params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
     process_without_stringified_params(action, params, session, flash, http_method)
   end
   alias_method_chain :process, :stringified_params