typescript-angular2 query string fix

[revaultch]

This should fix the following problems, I ran into:

1. typescript complaining: "Cannot create property 'xxx' on string '' " (string treated as map)
2. I have a subclass of RequestOptions which will call merge, which behind the scenes calls clone which isn't implemented on string

[wing328]

@Vrolijkx would you review the change when you've time? Thanks.

[Vrolijkx]

Looks ok for me if the build passes.
Didn't know URLSearchParams existed but it certainly makes the code look more robust.

[wing328]

@revaultch would you please rebase on the latest master to resolve the merge conflicts?

[wing328]

@revaultch thanks for your contribution.

[wing328]

@revaultch btw, I would suggest make the change in a new branch (as per Git best practices)