usability: publish fails with `api errors: not a valid url: ` if `homepage=""`

[Byron]

When publishing google-apis-rs, I found a bug in the generator which would produce Cargo.toml files like this:

[package]

name = "google-dataflow1_b4"
version = "0.1.3+20150401"
authors = ["Sebastian Thiel <byronimo@gmail>"]
description = "A complete library to interact with dataflow (protocol v1b4)"
repository = "https://github.com/Byron/google-apis-rs/tree/master/gen/dataflow1_b4"
homepage = ""
documentation = "http://byron.github.io/google-apis-rs/google-dataflow1_b4"
license = "MIT"
keywords = ["dataflow", "google", "protocol", "web", "api"]

[dependencies]
hyper = "*"
mime = "*"
yup-oauth2 = "*"
url = "*"
serde = "*"
serde_macros = "*"

Note the empty homepage string, which sneaked in there as the source material also contains an empty string field which wasn't explicitly excluded by me.

When publishing the file above one will see a relatively non-descript error like this:

cd gen/dataflow1_b4 && cargo publish
    Updating registry `https://github.com/rust-lang/crates.io-index`
   Uploading google-dataflow1_b4 v0.1.3+20150401 (file:///Users/byron/Documents/dev/rust/google-apis-rs/gen/dataflow1_b4)
api errors: not a valid url:

I think it would help if it would be explicit and say which field is empty, or ignore empty fields altogether.

Meta

➜  google-apis-rs git:(master) ✗ cargo --version --verbose
cargo 0.0.1-pre-nightly (3499c8e 2015-04-05) (built 2015-04-05)