Fix and simplify page url resolution in the default BasePageProvider

The previous logic fails if the resolution of a path's URL does not end with the path itself

e.g. if  `aria.core.DownloadMgr.resolveURL("foo.bar")` returns `"http://whatever/the/path?path=foo.bar&param=value"`

i.e. if the `DownloadMgr` has been customized (thanks to the `updateUrlMap(...)` function) to be redirected to a servlet with additional parameters (here the `'param=value'` appended to the url).

That's why I suggest this simplification including the removal of the variable `this._basePageUr'` made useless.