Added '_.ensureArray' function

[krawaller]

This pull request adds a _.ensureArray utility function, which we frequently found ourselves mixin into underscore. As the name implies, it helps when you want to feed a perhaps-array value into an API that expects an array:

// arrays are passed straight through
_.ensureArray([1,2,3]); // => [1,2,3]

// non-array truthy values are wrapped in an array
_.ensureArray({a:1,b:2}); // => [{a:1,b:2}]
_.ensureArray("foo"); // => ["foo"]
_.ensureArray(432); // => [432]

// falsy values result in an empty array
_.ensureArray(null); // => []

Semantically we are perhaps treading a bit too far into the land of _.toArray here, but the functions behave very differently since toArray always treats the argument as a collection. For the above use cases, here is what toArray returns:

_.toArray([1,2,3]); // => cloned array [1,2,3]
_.toArray({a:1,b:2}); // => [1,2]
_.toArray("foo"); // => ["f","o","o"]
_.toArray(432); // => []

Only for falsy values do the two functions act the same.

[michaelficarra]

[].concat(X) is shorter and doesn't require underscore.

[krawaller]

[].concat(x) comes frustratingly close to solve the use case, but since [].concat(undefinedvar) === [undefined] it isn't usable. If the value is undefined we don't want to make the concatenation, and thus we have to do the code branching that _.ensureArray sets out to eliminate.

[jashkenas]

Interesting, but way too special-case for Underscore. There are many situations where an API expects a particular type of object (that may or may not be coerceable from your input) ... and Arrays aren't the only type in town.