Create database in utf8 even if DATABASE_ENCODING is utf8mb4.

This is because we specify the `utf8mb4` charset in some of the columns
and expect others to be defaulted to `utf8`.

I'd like to make this a feature of `ar_mysql_column_charset`.  If you
want to set a table charset, you can always pass a keyword option
`options: "default charset=utf8mb4"` to `create_table`.

Complements #414.

lib/ar_mysql_column_charset.rb

@@ -63,6 +63,15 @@ module ActiveRecord::ConnectionAdapters
           end
         }
       end
+
+      def create_database(name, options = {})
+        # utf8mb4 is used in column definitions; use utf8 for
+        # databases.
+        if options[:charset] == 'utf8mb4'
+          options = options.merge(charset: 'utf8')
+        end
+        super(name, options)
+      end
     end
 
     prepend CharsetSupport