Do not assign `field` to itself if no description specified when deriving input object (#187)
Do not assign `field` to itself if no description specified when deriving input object (#187)

If a field has a description, we properly generate a meta field definition like:

```rust
{
  let field = registry.field::<String>("id", &());
  let field = field.description("Whatever");
  field
}
```

If a field does not have a description, we were generating something like:

```rust
{
  let field = registry.field::<String>("id", &());
  let field = field; // <--- Note not needed
  field
}
```

This of course works (and was likely optimized out by the compiler), but bloats
generated code for no benefit.

This change merely makes the second case generate:

```rust
{
  let field = registry.field::<String>("id", &());

  field
}
```

Fixes https://github.com/graphql-rust/juniper/issues/185.
mhallin/juniper,graphql-rust/juniper