rename is necessary to roundtrip, otherwise the rust name is used.
This also remove the rename if they are not necessary.
Note that:
```
#[serde(rename="foo", skip_serializing_if=="bar")]
pub field: bool,
```
is equivalent to:
```
#[serde(rename="foo")]
#[serde(skip_serializing_if=="bar")]
pub field: bool,
```
and for simplicity of construction the latter is used