Add $readPreference to spec when appropriate.

def _maybe_add_read_preference(spec, read_preference):
    """Add $readPreference to spec when appropriate."""
    mode = read_preference.mode
    tag_sets = read_preference.tag_sets
    max_staleness = read_preference.max_staleness
    # Only add $readPreference if it's something other than primary to avoid
    # problems with mongos versions that don't support read preferences. Also,
    # for maximum backwards compatibility, don't add $readPreference for
    # secondaryPreferred unless tags or maxStalenessSeconds are in use (setting
    # the slaveOkay bit has the same effect).
    if mode and (
        mode != ReadPreference.SECONDARY_PREFERRED.mode
        or tag_sets != [{}]
        or max_staleness != -1):

        if "$query" not in spec:
            spec = SON([("$query", spec)])
        spec["$readPreference"] = read_preference.document
    return spec