serde: skipping unused fields, don't check UTF-8

When the deserializer skips unused fields, we don't have to convert
them to valid Utf-8 string. This will prevent errors when unused fields
are not UTF-8. Removing the check may also improve performance.

See also: #198 

PR #203

src/deserializer.rs

@@ -557,7 +557,7 @@ impl<'a, 'de: 'a, T: DeRecord<'de>> Deserializer<'de>
  // Read and drop the next field.
  // This code is reached, e.g., when trying to deserialize a header
  // that doesn't exist in the destination struct.
- let _ = self.next_field()?;
+ let _ = self.next_field_bytes()?;
  visitor.visit_unit()
  }
 }