Fix warning: `*' interpreted as argument prefix
Merge pull request #18226 from kamipo/fix_warning_interpreted_as_argument_prefix

Fix warning: `*' interpreted as argument prefix
Add unsigned option support for MySQL numeric data types.

Example:

    create_table :foos do |t|
      t.integer :unsigned_integer, unsigned: true
      t.bigint  :unsigned_bigint,  unsigned: true
      t.float   :unsigned_float,   unsigned: true
      t.decimal :unsigned_decimal, unsigned: true, precision: 10, scale: 2
    end