Fix bigint? for Enum columns in MySQL

Follow up to #22896.

activerecord/lib/active_record/connection_adapters/column.rb

@@ -30,7 +30,7 @@ def has_default?
       end
 
       def bigint?
-        /bigint/ === sql_type
+        /\Abigint\b/ === sql_type
       end
 
       # Returns the human name of the column name.

activerecord/test/cases/adapters/mysql2/enum_test.rb

@@ -18,4 +18,9 @@ def test_should_not_be_unsigned
     column = EnumTest.columns_hash['enum_column']
     assert_not column.unsigned?
   end
+
+  def test_should_not_be_bigint
+    column = EnumTest.columns_hash['enum_column']
+    assert_not column.bigint?
+  end
 end

activerecord/test/schema/mysql2_specific_schema.rb

@@ -62,7 +62,7 @@
 
   ActiveRecord::Base.connection.execute <<-SQL
 CREATE TABLE enum_tests (
-  enum_column ENUM('text','blob','tiny','medium','long','unsigned')
+  enum_column ENUM('text','blob','tiny','medium','long','unsigned','bigint')
 )
 SQL
 end