float should coerce to Float

it seems to be ancient history that we slowly added support for Decimal, but never changed the coercion of plain "float" to be Float, not Numeric w/ decimals:

_type_map = {
    int: Integer(),
    float: Numeric(),
    bool: BOOLEANTYPE,
    decimal.Decimal: Numeric(),
    dt.date: Date(),
    dt.datetime: DateTime(),
    dt.time: Time(),
    dt.timedelta: Interval(),
    util.NoneType: NULLTYPE
}

So the more expensive Decimal is added into situations where plain float is used, and also on SQLite we of course get the warning as well as the wrong type back:

from sqlalchemy import create_engine, literal, select

e = create_engine("sqlite://")

print repr(e.scalar(select([literal(4.56)])))

#!


$ python test.py 
/home/classic/dev/sqlalchemy/lib/sqlalchemy/sql/sqltypes.py:596: SAWarning: Dialect sqlite+pysqlite does *not* support Decimal objects natively, and SQLAlchemy must convert from floating point - rounding errors and other issues may occur. Please consider storing Decimal numbers as strings or integers on this platform for lossless storage.
  'storage.' % (dialect.name, dialect.driver))
Decimal('4.5600000000')

ouch!

this is easy to fix, but may or may not have hard implications for current applications. Consider for 1.2 if possible.

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