a body property named default generates non-compiling code

[ahl]

Fixes #557

The problem was that we generated a builder like this:

            pub fn default<T>(mut self, value: T) -> Self
            where
                T: ::std::convert::TryInto<bool>,
                T::Error: ::std::fmt::Display,
            {
                self.default = value
                    .try_into()
                    .map_err(|e| format!("error converting supplied value for default: {}", e));
                self
            }

No problem there... until we tried to create the operation builder:

        pub fn new(client: &'a super::Client) -> Self {
            Self {
                client: client,
                body: Ok(types::builder::Foo::default()),
            }
        }

This PR changes the call to this:

        pub fn new(client: &'a super::Client) -> Self {
            Self {
                client: client,
                body: Ok(::std::default::Default::default()),
            }
        }