to_dict() fails to write boolean fields without default when value is False

When you have a message with a boolean value as follows:
```
message Patient {
bool isRnaAvailable = 1;
}
```
then an object like
```
patient = Patient()
patient.is_rna_available = np.random.choice([True, False], 1)[0]
```
fails to write the `is_rna_available` field when it is False when calling `patient.to_dict()`, this does not happen when the value is True.

The issue seems to be in line 824 of file `betterproto/__init__py` where it runs the following comparison `elif v != self._get_field_default(field, meta) or include_default_values:`. It should be something like 'elif v != None or include_default_values', otherwise when you set a field to the same value as the default value and do not set `include_default values` then it is not written. This may be also the case for other types.