Allow known attributes with customized namespace prefix on xliff elements

[arnoldbeiland]

Closes #33

Not checking the namespace prefix name seemed to be the solution for the bug linked.

However we also have to take care of cases where there are multiple attributes on an element with the same name (from different namespaces). See the test files from the commit:

• Xliff.OM.Tests/TestData.Xliff.OM.Tests/UnitWithAttributeHavingCustomNsPrefix.xlf (testing the issue)
• Xliff.OM.Tests/TestData.Xliff.OM.Tests/DocumentWithTwoNamespaces.xlf (testing a case with two version attributes on the xliff element; the one from the "its:" namespace should be considered an extension, while the other one should be set as a property of the corresponding XliffElement).

[msftclas]

All CLA requirements met.

[VladimirRybalko]

Hi @arnoldbeiland
Thank you very much for your PR.

Unfortunately, this project is dead. However, I created a new fork: Xliff.OM.NetStandar with all latest bugfixes including your changes.

The corresponding Nuget package is also updated.