Fix URi tests (and class) to be omcpatible with both 2.0 and 4.0 (and SL4)

Fix URi tests (and class) to be omcpatible with both 2.0 and 4.0 (and SL4)

* System/Uri.cs: Fix some "bugs" that are not part of FX4 anymore.
Change how Query/Fragment are merged to match FX4 (while keeping it
compatible with buggy FX2 under the NET_2_0 profile)

* Test/System/UriTest.cs: Adjust test cases to work with 4.0. Mark with
NotDotNet two test cases that are Mono/Unix-specific
* Test/System/UriTest2.cs: Add new test cases where a relative Uri is
merged with an absolute Uri (some changes in SL4/FX4 wrt FX2)
* Test/System/UriTest3.cs: Adjust for bugs fixed after FX 2.0.