Fix `extract_expression_for_virtual_column` for MariaDB

I'm not sure why `Mysql2VirtualColumnTest#test_schema_dumping` passed
previously. But now the test not pass at least in MariaDB 10.1.9.
I fixed the regexp to respect `COLLATE`.

```
% ARCONN=mysql2 be ruby -w -Itest test/cases/adapters/mysql2/virtual_column_test.rb -n test_schema_dumping
Using mysql2
Run options: -n test_schema_dumping --seed 7131

F

Finished in 0.466304s, 2.1445 runs/s, 4.2890 assertions/s.

  1) Failure:
Mysql2VirtualColumnTest#test_schema_dumping [test/cases/adapters/mysql2/virtual_column_test.rb:55]:
Expected /t\.virtual\s+"upper_name",\s+type: :string,\s+as: "UPPER\(`name`\)"$/i to match "# This file is auto-generated from the current state of the database. Instead\n# of editing this file, please use the migrations feature of Active Record to\n# incrementally modify your database, and then regenerate this schema definition.\n#\n# Note that this schema.rb definition is the authoritative source for your\n# database schema. If you need to create the application database on another\n# system, you should be using db:schema:load, not running all the migrations\n# from scratch. The latter is a flawed and unsustainable approach (the more migrations\n# you'll amass, the slower it'll run and the greater likelihood for issues).\n#\n# It's strongly recommended that you check this file into your version control system.\n\nActiveRecord::Schema.define(version: 0) do\n\n  create_table \"virtual_columns\", force: :cascade, options: \"ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci\" do |t|\n    t.string \"name\"\n    t.virtual \"upper_name\", type: :string, as: \n    t.virtual \"name_length\", type: :integer, as: \"LENGTH(`name`)\", stored: true\n  end\n\nend\n".

1 runs, 2 assertions, 1 failures, 0 errors, 0 skips
```

```
> select @@version;
+--------------------+
| @@version          |
+--------------------+
| 10.1.9-MariaDB-log |
+--------------------+
1 row in set (0.00 sec)
```

0 files changed, 0 insertions, 0 deletions