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.
Description attribute was available, but not actually used for derive
Object, InputObject and enum and the fields on input object.
Description is now properly set in each of those cases.
Also, tests for all three derives now properly verify the meta
information (name and description) for the derived GraphQLType.
