Use Python's repr() to emit float values

This fixes issue #101: RJ's dtoa() seems to have a weird behavior with particular floating
point values.

See also Tencent/rapidjson#1237.

rapidjson.cpp

@@ -2276,8 +2276,25 @@ dumps_internal(
             else
                 writer->RawValue("Infinity", 8, kNumberType);
         }
-        else
-            writer->Double(d);
+        else {
+            // The RJ dtoa() produces "strange" results for particular values, see #101:
+            // use Python's repr() to emit a raw value instead of writer->Double(d)
+
+            PyObject *dr = PyObject_Repr(object);
+
+            if (dr == NULL)
+                return false;
+
+            Py_ssize_t l;
+            const char* rs = PyUnicode_AsUTF8AndSize(dr, &l);
+            if (rs == NULL) {
+                Py_DECREF(dr);
+                return false;
+            }
+
+            writer->RawValue(rs, l, kNumberType);
+            Py_DECREF(dr);
+        }
     }
     else if (PyUnicode_Check(object)) {
         Py_ssize_t l;

tests/test_base_types.py

@@ -30,6 +30,13 @@ def test_base_values(value, dumps, loads):
     assert loaded == value and type(loaded) is type(value)
 
 
+@pytest.mark.unit
+def test_float(dumps):
+    value = 0.1 + 0.2
+    dumped = dumps(value)
+    assert dumped == '0.30000000000000004'
+
+
 @pytest.mark.unit
 def test_tuple(dumps):
     obj = [1, 2, 'a', 1.2, {'foo': 'bar'},]