Extend PlaceRequest.Builder to build from an existing PlaceRequest

[ryankenney]

The com.gwtplatform.mvp.client.proxy.PlaceRequest.Builder implementation in GWTP 1.0 doesn't allow you to build from an existing PlaceRequest. Our common use case is to query com.gwtplatform.mvp.client.proxy.PlaceManager.getCurrentPlaceRequest() and then
add/change something.

I propose a new constructor for PlaceRequest.Builder:

PlaceRequest.Builder(PlaceRequest request)

[branflake2267]

Thanks for reporting the issue.

[bmoritz]

Opened PR #304 to enhance PlaceRequest Builder accordingly

[christiangoudreau]

Please update documentation to include your new feature: https://github.com/ArcBees/GWTP/wiki/URL-Parameters

Feel free to improve if you find something to improve :D (Boy scout rule)

[bmoritz]

Done

[branflake2267]

Good job! :)