Add failing test to demonstrate issue

From the
[docs](https://docs.pydantic.dev/latest/concepts/fields/#exclude):

> The `exclude` parameter can be used to control which fields should
> be excluded from the model when exporting the model.

At the moment the values are excluded as expected, but the headers are
not correctly filtered. This is worse than we expect because it also
breaks the order of the fields rather than leaving the field blank.

Author: Henry Walshaw

tests/models.py

@@ -51,3 +51,8 @@ def parse_start_date(cls, value):
     @pydantic.field_validator("end", mode="before")
     def parse_end_date(cls, value):
         return datetime.strptime(value, "%d.%m.%Y").date()
+
+
+class ExcludedPassword(pydantic.BaseModel):
+    username: str = "Wagstaff"
+    password: str = Field(default="swordfish", exclude=True)

tests/test_basemodel_csv_writer.py

@@ -4,7 +4,7 @@
 
 from pydantic_csv import BasemodelCSVWriter
 
-from .models import NonBaseModelUser, SimpleUser, User
+from .models import ExcludedPassword, NonBaseModelUser, SimpleUser, User
 
 
 def test_create_csv_file(users_as_csv_buffer, users_from_csv):
@@ -50,3 +50,13 @@ def test_with_wrong_type_in_list(user_list):
 
 def test_header_mapping(users_mapped_as_csv_buffer, users_mapped_from_csv):
     assert users_mapped_as_csv_buffer == users_mapped_from_csv
+
+
+def test_excluded_field():
+    output = io.StringIO()
+    user = ExcludedPassword()
+
+    w = BasemodelCSVWriter(output, [user], ExcludedPassword)
+    w.write()
+
+    assert output.getvalue() == "username\r\nWagstaff\r\n"