Steps to reproduce:

1) Download attached project 'UnityUriBug.zip' and open in Unity.
2) In toolbar click on Tools >> 'UriTest'.
3) Enter "http://www.contoso.com/" for Path 1 and "http://www.contoso.com/test/test.htm" for Path 2(naturally, don't include the quotes).
4) Press the button "Update Relative".
Note: The correct relative path ("test/test.htm") is printed in the field below the button.
5) Now enter "http://www.contoso.com/test1/" for Path 1 and "http://www.contoso.com/" for Path 2.
6) Press the button "Update Relative".
Note: The text field incorrectly prints the empty string when it should print "../". ( see more details in https://msdn.microsoft.com/en-us/library/system.uri.makerelativeuri(v=vs.110).aspx )

Expected result: In the tab 'Path 2 relative to Path 1' should be printed "../".

Actual result: Tab 'Path 2 relative to Path 1' is empty.

Reproduced with: 5.2.4f1, 5.3.6f1, 5.4.0f2, 5.5.0a4.

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This case will be addressed as part of Mono runtime upgrade
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