`valid_type?` should accept only supported types

`valid_type?` is used in schema dumper to determine if a type is
supported. So if `valid_type?(:foobar)` is true, it means that schema
dumper is allowed to create `t.foobar`. But it doesn't work. I think
that `valid_type?` should accept only supported types.

https://github.com/rails/rails/blob/v5.1.0.beta1/activerecord/lib/active_record/schema_dumper.rb#L135-L142

```ruby
  columns.each do |column|
    raise StandardError, "Unknown type '#{column.sql_type}' for column '#{column.name}'" unless @connection.valid_type?(column.type)
    next if column.name == pk
    type, colspec = @connection.column_spec(column)
    tbl.print "    t.#{type} #{column.name.inspect}"
    tbl.print ", #{format_colspec(colspec)}" if colspec.present?
    tbl.puts
  end
```

1 files changed, 0 insertions, 11 deletions

diff --git a/activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb b/activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb
index aab3dcb724..565130c38f 100644
--- a/activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb
+++ b/activerecord/test/cases/adapters/mysql2/mysql2_adapter_test.rb
@@ -17,17 +17,6 @@ class Mysql2AdapterTest < ActiveRecord::Mysql2TestCase
     end
   end
 
-  def test_valid_column
-    with_example_table do
-      column = @conn.columns("ex").find { |col| col.name == "id" }
-      assert @conn.valid_type?(column.type)
-    end
-  end
-
-  def test_invalid_column
-    assert_not @conn.valid_type?(:foobar)
-  end
-
   def test_columns_for_distinct_zero_orders
     assert_equal "posts.id",
       @conn.columns_for_distinct("posts.id", [])