Addition with zero yields BigInt parsing errors

Created by: xTibor

Reproduction:

extern crate calc;

fn main() {
    let expressions = &["1+0", "0+1", "0+0", "(1+0)+0"];

    for e in expressions.iter() {
        println!("{} = {:?}", e, calc::eval(e));
    }
}

Expected behavior:

1+0 = Ok(Integral(BigInt { sign: Plus, data: BigUint { data: [1] } }, Dec))
0+1 = Ok(Integral(BigInt { sign: Plus, data: BigUint { data: [1] } }, Dec))
0+0 = Ok(Integral(BigInt { sign: Plus, data: BigUint { data: [0] } }, Dec))
(1+0)+0 = Ok(Integral(BigInt { sign: Plus, data: BigUint { data: [1] } }, Dec))

Actual behavior:

1+0 = Ok(Integral(BigInt { sign: Plus, data: BigUint { data: [1] } }, Dec))
0+1 = Err(InvalidNumber("failed to parse bigint/biguint"))
0+0 = Err(InvalidNumber("failed to parse bigint/biguint"))
(1+0)+0 = Err(InvalidNumber("failed to parse bigint/biguint"))

Build information: rustc -V: rustc 1.28.0-nightly (5d0631a64 2018-05-30) git rev-parse HEAD: 90c85a7e System: KDE neon 5.12 (Ubuntu 16.04)