#1660: Fix ExcelControl copying of stringified attribute (#1661)

# Description

As described in #1660, the `stringified` attribute is not copied during
the `to_copy()` as it is not included in the `metadata_profile_patch`.

This commit fixes it by adding it to the `metadata_profile_patch` and
updates the documentation to clarify the interaction of `stringified`
and schemas. This also inclues a unit test to verify the bug fix.

- fixes #1660 
- fixes #1659 

# Tests

Ensure the new unit test detects the issue before the fix:

```
> pytest frictionless/formats/excel/__spec__/test_control.py::test_excel_control_to_copy

[...]

>       assert control_copy == control_with_changed_attributes
E       AssertionError: assert {'type': 'exc...tError': True} == {'type': 'exc...tError': True}
E
E         Omitting 8 identical items, use -vv to show
E         Differing attributes:
E         ['stringified']
E
E         Drill down into differing attribute stringified:
E           stringified: False != True

frictionless/formats/excel/__spec__/test_control.py:25: AssertionError
```

Ensure the unit test runs and passes after the fix:

```
> pytest frictionless/formats/excel/__spec__/test_control.py::test_excel_control_to_copy -v

[...]

frictionless/formats/excel/__spec__/test_control.py::test_excel_control_to_copy PASSED [100%]
```

frictionless/formats/excel/__spec__/test_control.py

@@ -6,3 +6,20 @@
 def test_excel_dialect():
     with Resource("data/table.xlsx") as resource:
         assert isinstance(resource.dialect.get_control("excel"), formats.ExcelControl)
+
+def test_excel_control_to_copy():
+    """
+    Test that the ExcelControl and all its attributes are correctly copied
+    """
+    # Make a control with all values changed from the defaults
+    control_with_changed_attributes = formats.ExcelControl(
+        sheet="non-default",
+        fill_merged_cells=True,
+        preserve_formatting=True,
+        adjust_floating_point_error=True,
+        stringified=True
+    )
+
+    control_copy = control_with_changed_attributes.to_copy()
+
+    assert control_copy == control_with_changed_attributes

frictionless/formats/excel/control.py

@@ -51,6 +51,12 @@ class ExcelControl(Control):
     """
     Stringifies all the cell values. Default value
     is False.
+
+    Note that a table resource schema will still be applied and types coerced to match the schema
+    (either provided or inferred) _after_ the rows are read as strings.
+
+    To return all cells as strings then both set `stringified=True` and specify a 
+    schema that defines all fields to be of type string (see #1659).
     """
 
     # Metadata
@@ -61,5 +67,6 @@ class ExcelControl(Control):
             "fillMergedCells": {"type": "boolean"},
             "preserveFormatting": {"type": "boolean"},
             "adjustFloatingPointError": {"type": "boolean"},
+            "stringified": {"type": "boolean"},
         },
     }