Make ProcessorFormatter able to log dict (#189)

Closes #188,

src/structlog/stdlib.py

@@ -451,7 +451,7 @@ def format(self, record):
         # Make a shallow copy of the record to let other handlers/formatters
         # process the original one
         record = logging.makeLogRecord(record.__dict__)
-        if isinstance(record.msg, dict):
+        try:
             # Both attached by wrap_for_formatter
             logger = record._logger
             meth_name = record._name
@@ -460,7 +460,7 @@ def format(self, record):
             # processed by multiple logging formatters.  LogRecord.getMessage
             # would transform our dict into a str.
             ed = record.msg.copy()
-        else:
+        except AttributeError:
             logger = None
             meth_name = record.levelname.lower()
             ed = {"event": record.getMessage(), "_record": record}

tests/test_stdlib.py

@@ -495,7 +495,7 @@ def test_foreign_delegate(self, configure_for_pf, capsys):
 
         assert ("", "foo [in test_foreign_delegate]\n") == capsys.readouterr()
 
-    def test_clears_args(self, capsys, configure_for_pf):
+    def test_clears_args(self, configure_for_pf, capsys):
         """
         We render our log records before sending it back to logging.  Therefore
         we must clear `LogRecord.args` otherwise the user gets an
@@ -511,6 +511,19 @@ def test_clears_args(self, capsys, configure_for_pf):
             "hello world. [in test_clears_args]\n",
         ) == capsys.readouterr()
 
+    def test_log_dict(self, configure_for_pf, capsys):
+        """
+        Test that dicts can be logged with std library loggers.
+        """
+        configure_logging(None)
+
+        logging.getLogger().warning({"foo": "bar"})
+
+        assert (
+            "",
+            "{'foo': 'bar'} [in test_log_dict]\n",
+        ) == capsys.readouterr()
+
     def test_foreign_pre_chain(self, configure_for_pf, capsys):
         """
         If foreign_pre_chain is an iterable, it's used to pre-process