Insert default values behaviour

[tyt2y3]

I totally think that the above API behaviour is too smart, the following should be better.
There is no basis that the query builder just assumes that DEFAULT VALUES is valid for the table either.

assert_eq!(
    Query::insert()
        .into_table(Glyph::Table)
        .or_default_values()
        .to_string(PostgresQueryBuilder),
    "INSERT INTO \"glyph\" DEFAULT VALUES"
);
assert_eq!(
    Query::insert()
        .into_table(Glyph::Table)
        .columns(vec![Glyph::Image])
        .values_panic(vec!["ABC".into()])
        .or_default_values()
        .to_string(PostgresQueryBuilder),
    "INSERT INTO \"glyph\" (\"image\") VALUES ('ABC')"
);

Originally posted by @tyt2y3 in #223 (comment)

[tyt2y3]

Reverted #223 for now. We can include this feature in the next next release as I want to make a release now.