Add Decimal codec tests

Ensure it both unwraps as a `String` (or `str`) and that the
`api.decimal()` encoder routine does the same.

tests/test_api.py

@@ -187,6 +187,12 @@ def test_integer():
     assert isinstance(i, Integer)
 
 
+def test_decimal():
+    d = tomlkit.decimal("34.56")
+
+    assert isinstance(d, String)
+
+
 def test_float():
     i = tomlkit.float_("34.56")
 

tests/test_items.py

@@ -2,6 +2,7 @@
 import math
 import pickle
 
+from decimal import Decimal
 from datetime import date
 from datetime import datetime
 from datetime import time
@@ -90,6 +91,16 @@ def test_integer_unwrap():
     elementary_test(item(666), int)
 
 
+def test_decimal_unwrap():
+    """Ensure a decimal unwraps as a string
+    after TOML encode.
+    """
+    elementary_test(
+        item(Decimal("0.001")),
+        str,
+    )
+
+
 def test_float_unwrap():
     elementary_test(item(2.78), float)