Fix #400 - Preserve POM content ordering (and formatting) on resolveCiFriendliesOnly flattenMode

[stechio]

As reported in #400, resolveCiFriendliesOnly flattenMode doesn't preserve POM content ordering, formatting or comments (KeepCommentsInPom is just a less-than-ideal workaround which quickly falls apart when comment nodes relate to sections rather than single elements); this is typically inconvenient, as users who don't apply other transformations may legitimately expect the output being identical to the input, interpolation aside.

This fix provides such functionality, interpolating the original POM without interfering with its formatting, ordering or comments. Leveraging the simple placeholder syntax (which doesn't require deserialization for modifications), the original POM content string is processed for interpolation as a whole.

En passant, this PR possibly fixes also #401 as, debugging the main fix, CiInterpolatorImpl.interpolate(...) initially behaved weirdly, assigning a spurious value to unresolved placeholders (any expression other than CI-friendly properties was resolved to its name (eg, "${test.version}" -> "test.version")?!):

Object value = existingAnswers.get(realExpr);
if (!wholeExpr.equals("${revision}")
        && !wholeExpr.contains("${sha1}")
        && !wholeExpr.contains("${changelist}")) {
    value = realExpr;
}

With this fix, any unknown placeholder is duly left unresolved:

Object value = null;
if (wholeExpr.equals("${revision}")
        || wholeExpr.contains("${sha1}")
        || wholeExpr.contains("${changelist}")) {
    value = existingAnswers.get(realExpr);
    . . .
}

Additionally, toString() method of PomProperty and CacheField has been overridden to improve debugging intelligibility.

Testing

• KeepCommentsInPomTest resources have been updated to pass the new implementation
• resolveCiFriendliesOnly-related integration tests initially failed, until resolveCiFriendliesOnly garbles non ci-friendly property-placeholder if used in the same property #401-related code was fixed in turn