Add Seq.toMap() with mergeFunction

[mujun0312]

As the title, the method toMap() throws an exception by default when the keys are the same, whether to overload method toMap() by a merge function, used to resolve collisions between values associated with the same key.
The version i used is 0.9.12.

[lukaseder]

Thanks for your suggestion.

I see, we could offer this, because Collector also offers a version that has a BinaryOperator<U> mergeFunction, and our own Collectable.toMap() methods are merely convenience for the Collector version.

Of course, the workaround is to use the Collector API from the JDK