ListAttachmentArchive's toRCS() method modifies the revision dates

This issue was introduced in XWIKI-5942.

To summarize the problem:

• toRCS() will call new Archive() or addRevision() which create new JRCS Node objects (either TrunkNode or BranchNode).
• When a new JRCS Node is created, the date of the node is set to the current date (and time)
• Thus calling toRCS() generates an Archive object that doesn't contain the real dates of the attachment revisions but the date of when toRCS() was called.

This affects the new ListAttachmentArchiveTest#testGetArchiveAsString() test which compares ListAttachmentArchive#getArchive() (which calls toRCS()) with XWikiAttachmentArchive#getArchive(). The later is correct and doesn't change the dates but the former does and thus if the test doesn't execute fast enough, the comparison assert fails with:

org.opentest4j.AssertionFailedError: array contents differ at index [85], expected: <51> but was: <50>
	at org.junit.jupiter.api.AssertionUtils.fail(AssertionUtils.java:39)
	at org.junit.jupiter.api.AssertArrayEquals.failArraysNotEqual(AssertArrayEquals.java:432)
	at org.junit.jupiter.api.AssertArrayEquals.assertArrayEquals(AssertArrayEquals.java:212)
	at org.junit.jupiter.api.AssertArrayEquals.assertArrayEquals(AssertArrayEquals.java:66)
	at org.junit.jupiter.api.AssertArrayEquals.assertArrayEquals(AssertArrayEquals.java:62)
	at org.junit.jupiter.api.Assertions.assertArrayEquals(Assertions.java:1208)
	at org.xwiki.store.legacy.doc.internal.ListAttachmentArchiveTest.testGetArchiveAsString(ListAttachmentArchiveTest.java:157)

One solution is to extend Archive (see XWikiRCSArchive) with a new XWikiAttachmentRCSArchive class for example and have the code get the date from the attachment XML data and after the Node has been created, set the correct date on it.