Support capital letters

The tokenizer has a different output than the one in Python.

For the text "(CNN)When"

Result [75, 29, 29, 15832]

Should be [41, 254, 17235, 61, 10555, 1]

Dart code
```dart
String getLongText = """(CNN)When""";
Uint32List encodedList = tiktoken.encode(getLongText);

print('encodedList');
print(encodedList);
```

Python code
```Python
ARTICLE = """(CNN)When"""
inputs = tokenizer(ARTICLE, return_tensors="pt", padding=True, truncation=True)
print(inputs['input_ids'])
```