Remove error throwing in _.reduce & _.reduceRight.

[jdalton]

With the use of ES5 array built-ins removed from Underscore there's no longer a need to throw an error when a collection is empty and no initial value is provided in _.reduce and _.reduceRight. Thoughts?

// currently
_.reduce([], _.identity, 'a'); // => 'a'
_.reduce([], _.identity); // => throws error

// instead it could simply do:
_.reduce([], _.identity); // => undefined

This would align with changes to _.keys which no longer throws either.

[braddunbar]

👍

[megawac]

👍 for #1841

[jashkenas]

Sounds great.

[yy8597]

so nice