Fix IContentBase.GetUdi() extension method to support document-blueprint entity type

[ronaldbarendse]

Prerequisites

• I have added steps to test this contribution in the description below

Description

While implementing document blueprint container support in Deploy 14, I noticed document blueprints (IContent with Blueprint = true) were returning UDIs using the document entity type (e.g. umb://document/{GUID}, instead of umb://document-blueprint/{GUID}).

This PR adds tests for all supported entity types, fixes the issue and cleans up the UdiGetterExtensions class to remove redundant code. The added tests should be sufficient to verify the issue, but reverting to the first commit will show the failing test:

Expected string length 57 but was 47. Strings differ at index 14.
Expected: "umb://document-blueprint/6ad82c70685c4e049b36d81bd779d16f"
But was:  "umb://document/6ad82c70685c4e049b36d81bd779d16f"
-------------------------^

[AndyButland]

Changes look good to me and can see the passing tests have verified the issue.

Please make sure you contact one of the CMS team directly (Lasse, Bjarke) just to make sure this doesn't fall between the HQ and community teams.