Fix #74: typing of GroupBy overload with result selector.

[ergwun]

This pull request is for a fix for issue #74.

I have fixed the typing of the GroupBy overload with the result selector, and added a unit test to show successful assignation of that overload's result to an explicitly-typed variable.

It is also applicable to the linq-es5 branch, but I presume that would be handled independently.

[coveralls]

Coverage remained the same at 99.804% when pulling 6b711bc on ergwun:issue74 into e098395 on ENikS:master.

[ENikS]

I am not sure this is the best approach.
@MgSam tried to improve IGrouping in this commit but I guess he did not cover all the cases. Perhaps you could do something alone these lines?

[ergwun]

I don't think it is a problem with IGrouping, as I do not believe that the third overload should be returning a grouping at all.

I'm basing this on the return types of the various GroupBy overloads in C#. Note how the first two overloads return enumerables of groupings, but the one with the result selector returns an enumerable of the type returned by the result selector:

public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(
	this IEnumerable<TSource> source,
	Func<TSource, TKey> keySelector
)

public static IEnumerable<IGrouping<TKey, TElement>> GroupBy<TSource, TKey, TElement>(
	this IEnumerable<TSource> source,
	Func<TSource, TKey> keySelector,
	Func<TSource, TElement> elementSelector
)

public static IEnumerable<TResult> GroupBy<TSource, TKey, TResult>(
	this IEnumerable<TSource> source,
	Func<TSource, TKey> keySelector,
	Func<TKey, IEnumerable<TSource>, TResult> resultSelector
)

[MgSam]

@ergwun Good catch, my mistake. Could you update the linq-es5 branch as well? (It's actually a different package on NPM)