Field name and line numbers are missing in internally tagged enum

Here is a test case that shows the failure. Comment/uncomment one of the `let json_deserializer =` lines to toggle between the JSON test data. The output for the failing case is:
```
Error parsing field: ., at position 0:0
invalid type: string "500", expected u32
```

```rs
#![feature(proc_macro_hygiene, decl_macro)]

use serde_derive::Deserialize;

#[derive(Debug, Deserialize)]
#[serde(tag = "type", content = "content")]
pub enum TestStruct {
A(u32),
}

#[derive(Debug, Deserialize)]
#[serde(tag = "type")]
pub enum Wrapper {
B { value: TestStruct },
}

#[allow(dead_code)]
const WORKING_JSON: &'static str = r#"
{
"type": "B",
"value": { "type": "A", "content": 500 }
}"#;

#[allow(dead_code)]
const FAILING_FIELD_NAME_AND_LINE: &'static str = r#"
{
"type": "B",
"value": { "type": "A", "content": "500" }
}"#;

fn main() {
// Example of JSON deserialization error, speficially a string is provided instead of u32.
// Comment out one of these two lines to see the difference between the working and failing JSON
let json_deserializer = &mut serde_json::Deserializer::from_str(WORKING_JSON);
// let json_deserializer = &mut serde_json::Deserializer::from_str(FAILING_FIELD_NAME_AND_LINE);

let result: Result = serde_path_to_error::deserialize(json_deserializer);
match result {
Ok(e) => println!("{:?}", e),
Err(e) => {
let path = e.path().to_string();
let i = e.into_inner();
println!(
"Error parsing field: {}, at position {}:{}\n{}",
path,
i.line(),
i.column(),
i
)
}
}
}
```