`max_decimal_places` in Decimal field are wrong calculated

[bimusiek]

We got an issue when number is formatted as decimal.Decimal('2E+9').

How DecimalField counts decimals:

    sign, digittuple, exponent = value.as_tuple()
    decimals = abs(exponent)

However result of decimal.Decimal('2E+9').as_tuple()[2] is 9, which is ok, but there are no decimal places in this number.

My solution is to not do abs and instead multiply by -1.

I can prepare PR tonight if you think it is valid.

[lovelydinosaur]

So things that would help here:

• Any comparisons between what happens in Django core and what we do (if that's possible/relevant?)
• One or two example cases (In particular pull requests with failing test cases), and expected input/output values.

[bimusiek]

@tomchristie I added PR with example and solution.
Basically we get really huge number, then normalize it with .normalize() which resulted in having 2E+9 which resulted in ValidationError :)

Hope PR is valid!