Do not trim element content for string leaf values

Do not trim element content for string leaf values

Trimming element text leads to values being mis-represented -- for
example if the content is a YANG string, we really need to pick it up
directly.

We should completely avoid trimming, but that would lead to not
recognizing previously-accepted XML documents, for example those which
correspond to a uint8 with leading whitespace.

JIRA: YANGTOOLS-1522
Change-Id: Ib1e05a9428a66765b2f85cfde723fe27076f68ee
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>