Add `SchemaMigration.create_table` support any unicode charsets for MySQL.

MySQL unicode support is not only `utf8mb4`.
Then, The index length problem is not only `utf8mb4`.

http://dev.mysql.com/doc/refman/5.6/en/charset-unicode.html

    SELECT * FROM information_schema.character_sets WHERE maxlen > 3;
    +--------------------+----------------------+------------------+--------+
    | CHARACTER_SET_NAME | DEFAULT_COLLATE_NAME | DESCRIPTION      | MAXLEN |
    +--------------------+----------------------+------------------+--------+
    | utf8mb4            | utf8mb4_general_ci   | UTF-8 Unicode    |      4 |
    | utf16              | utf16_general_ci     | UTF-16 Unicode   |      4 |
    | utf16le            | utf16le_general_ci   | UTF-16LE Unicode |      4 |
    | utf32              | utf32_general_ci     | UTF-32 Unicode   |      4 |
    +--------------------+----------------------+------------------+--------+

2 files changed, 10 insertions, 9 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index 98009da2bd..c084431588 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -235,6 +235,16 @@ module ActiveRecord
         end
       end
 
+      MAX_INDEX_LENGTH_FOR_CHARSETS_OF_4BYTES_MAXLEN = 191
+      CHARSETS_OF_4BYTES_MAXLEN = ['utf8mb4', 'utf16', 'utf16le', 'utf32']
+      def initialize_schema_migrations_table
+        if CHARSETS_OF_4BYTES_MAXLEN.include?(charset)
+          ActiveRecord::SchemaMigration.create_table(MAX_INDEX_LENGTH_FOR_CHARSETS_OF_4BYTES_MAXLEN)
+        else
+          ActiveRecord::SchemaMigration.create_table
+        end
+      end
+
       # Returns true, since this connection adapter supports migrations.
       def supports_migrations?
         true
diff --git a/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb b/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
index fac6f81540..21631be25c 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb
@@ -37,15 +37,6 @@ module ActiveRecord
         configure_connection
       end
 
-      MAX_INDEX_LENGTH_FOR_UTF8MB4 = 191
-      def initialize_schema_migrations_table
-        if charset == 'utf8mb4'
-          ActiveRecord::SchemaMigration.create_table(MAX_INDEX_LENGTH_FOR_UTF8MB4)
-        else
-          ActiveRecord::SchemaMigration.create_table
-        end
-      end
-
       def supports_explain?
         true
       end