Insert builder: fix mutually exclusive methods

[belchior]

Mutually exclusive methods

The methods Insert::values/Insert::row, Insert::select and Insert::set are mutually exclusive, the last called will overrides the previous ones. Insert::row and Insert::set are available on crate feature mysql only.

Example

#[cfg(feature = "mysql")]
{
  use sql_query_builder as sql;

  let query = sql::Insert::new()
    .values("('foo', 'Foo')")
    .row("('bar', 'Bar')")
    .set("login = 'foo'")
    .set("name = 'Foo'")
    .as_string();

  let expected = "SET login = 'foo', name = 'Foo'";

  assert_eq!(expected, query);
}

Output

SET login = 'foo', name = 'Foo'