Fix SPARQL CSV result serialization of blank nodes

This patch fixes SPARQL CSV result serialization of blank nodes.
Previously blank nodes were serialized the same as normal nodes, but
blank nodes should be prefixed with `_:` as per
<https://www.w3.org/TR/sparql11-results-csv-tsv/#csv-terms>:

> Blank nodes use the `_:label` form from Turtle and SPARQL.
> Use of the same label indicates the same blank node within
> the results but has no significance outside the results.

rdflib/plugins/sparql/results/csvresults.py

@@ -83,5 +83,7 @@ def serialize(self, stream: IO, encoding: str = "utf-8", **kwargs):
     def serializeTerm(self, term, encoding):
         if term is None:
             return ""
+        elif isinstance(term, BNode):
+            return f"_:{term}"
         else:
             return term

test/test_sparql/test_result.py

@@ -15,6 +15,7 @@
     Iterator,
     Mapping,
     Optional,
+    Pattern,
     Sequence,
     Set,
     TextIO,
@@ -28,8 +29,9 @@
 from pyparsing import ParseException
 
 from rdflib.graph import Graph
+from rdflib.namespace import Namespace
 from rdflib.query import Result, ResultRow
-from rdflib.term import Identifier, Literal, Node, Variable
+from rdflib.term import BNode, Identifier, Literal, Node, Variable
 
 BindingsType = Sequence[Mapping[Variable, Identifier]]
 ParseOutcomeType = Union[BindingsType, Type[Exception]]
@@ -74,6 +76,43 @@ def test_select_result_parse(
         assert parse_outcome == parsed_result.bindings
 
 
+EGSCHEME = Namespace("example:")
+
+
+@pytest.mark.parametrize(
+    ("node", "format", "expected_result"),
+    [
+        (BNode(), "csv", re.compile(r"^_:.*$")),
+        (BNode("a"), "csv", "_:a"),
+        (Literal("x11"), "csv", "x11"),
+    ],
+)
+def test_xsv_serialize(
+    node: Identifier, format: str, expected_result: Union[Pattern[str], str]
+) -> None:
+    graph = Graph()
+    graph.add((EGSCHEME.checkSubject, EGSCHEME.checkPredicate, node))
+    result = graph.query(
+        f"""
+    PREFIX egscheme: <{EGSCHEME}>
+    SELECT ?o {{
+        egscheme:checkSubject egscheme:checkPredicate ?o
+    }}
+    """
+    )
+    assert len(result.bindings) == 1
+    with BytesIO() as bio:
+        result.serialize(bio, format=format)
+        result_text = bio.getvalue().decode("utf-8")
+    result_lines = result_text.splitlines()
+    assert len(result_lines) == 2
+    logging.debug("result_lines[1] = %r", result_lines[1])
+    if isinstance(expected_result, str):
+        assert expected_result == result_lines[1]
+    else:
+        assert expected_result.match(result_lines[1])
+
+
 @pytest.fixture(scope="module")
 def select_result(rdfs_graph: Graph) -> Result:
     query = """