Don't treat begin and rollback transactions as write queries

Otherwise `save` method would raise the `ReadOnlyError` against `BEGIN`
and `ROLLBACK` queries.

activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb

@@ -19,7 +19,7 @@ def query(sql, name = nil) # :nodoc:
           execute(sql, name).to_a
         end
 
-        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :select, :set, :show, :release, :savepoint) # :nodoc:
+        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :select, :set, :show, :release, :savepoint, :rollback) # :nodoc:
         private_constant :READ_QUERY
 
         def write_query?(sql) # :nodoc:

activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb

@@ -67,7 +67,7 @@ def query(sql, name = nil) #:nodoc:
           end
         end
 
-        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:select, :show, :set) # :nodoc:
+        READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :select, :set, :show, :savepoint, :rollback) # :nodoc:
         private_constant :READ_QUERY
 
         def write_query?(sql) # :nodoc:

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -209,7 +209,7 @@ def disable_referential_integrity # :nodoc:
       # DATABASE STATEMENTS ======================================
       #++
 
-      READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:select, :pragma) # :nodoc:
+      READ_QUERY = ActiveRecord::ConnectionAdapters::AbstractAdapter.build_read_query_regexp(:begin, :select, :pragma, :savepoint, :rollback) # :nodoc:
       private_constant :READ_QUERY
 
       def write_query?(sql) # :nodoc:

activerecord/test/cases/base_test.rb

@@ -1490,31 +1490,37 @@ def test_protected_environments_are_stored_as_an_array_of_string
   end
 
   test "creating a record raises if preventing writes" do
-    assert_raises ActiveRecord::ReadOnlyError do
+    error = assert_raises ActiveRecord::ReadOnlyError do
       ActiveRecord::Base.connection.while_preventing_writes do
         Bird.create! name: "Bluejay"
       end
     end
+
+    assert_match %r/\AWrite query attempted while in readonly mode: INSERT /, error.message
   end
 
   test "updating a record raises if preventing writes" do
     bird = Bird.create! name: "Bluejay"
 
-    assert_raises ActiveRecord::ReadOnlyError do
+    error = assert_raises ActiveRecord::ReadOnlyError do
       ActiveRecord::Base.connection.while_preventing_writes do
         bird.update! name: "Robin"
       end
     end
+
+    assert_match %r/\AWrite query attempted while in readonly mode: UPDATE /, error.message
   end
 
   test "deleting a record raises if preventing writes" do
     bird = Bird.create! name: "Bluejay"
 
-    assert_raises ActiveRecord::ReadOnlyError do
+    error = assert_raises ActiveRecord::ReadOnlyError do
       ActiveRecord::Base.connection.while_preventing_writes do
         bird.destroy!
       end
     end
+
+    assert_match %r/\AWrite query attempted while in readonly mode: DELETE /, error.message
   end
 
   test "selecting a record does not raise if preventing writes" do

activerecord/test/models/bird.rb

@@ -6,6 +6,11 @@ class Bird < ActiveRecord::Base
 
   accepts_nested_attributes_for :pirate
 
+  before_save do
+    # force materialize_transactions
+    self.class.connection.materialize_transactions
+  end
+
   attr_accessor :cancel_save_from_callback
   before_save :cancel_save_callback_method, if: :cancel_save_from_callback
   def cancel_save_callback_method