Don’t convert Excel reports to CSV before output

Converting Python data to CSV makes every field a string. This means
that in the report we return to the user every field will be a string,
even if it’s come from an `int` type in Python. This is because the CSV
‘standard’ doesn’t support any kind of typing.

Excel does support types for fields, so we can make our reports more
useful by preserving these types. This is particularly relevant in the
report we generate for performance platform, which needs the `count`
column to be a number type.

This commit adds extra code paths to the `Spreadsheet` class which mean
that it can be instantiated from either CSV data or a list of Python
data. Previously we were converting the Python data to CSV as an
intermediate step, before instantiating the class.

tests/app/test_utils.py

@@ -140,6 +140,22 @@ def test_can_create_spreadsheet_from_dict_with_filename():
     assert Spreadsheet.from_dict({}, filename='empty.csv').as_dict['file_name'] == "empty.csv"
 
 
+@pytest.mark.parametrize('args, kwargs', (
+    (
+        ('hello', ['hello']),
+        {},
+    ),
+    (
+        (),
+        {'csv_data': 'hello', 'rows': ['hello']}
+    ),
+))
+def test_spreadsheet_checks_for_bad_arguments(args, kwargs):
+    with pytest.raises(TypeError) as exception:
+        Spreadsheet(*args, **kwargs)
+    assert str(exception.value) == 'Spreadsheet must be created from either rows or CSV data'
+
+
 @pytest.mark.parametrize('created_by_name, expected_content', [
     (
         None, [