Number filter with exponentially small numbers without fractionSize

Currently, if `fractionSize` is not passed, for small numbers like `1e-50` number filter produces string `1e-50`.

It happens because `fractionSize` is undefined here: https://github.com/angular/angular.js/blob/g3_v1_3/src/ng/filter/filters.js#L152

`undefined + 1` is `NaN` and `NaN` can't be bigger than number.

Fraction size is only computed if number has no exponent.

Is it by design?
