docopt parsing is too lenient with optional params

[debris]

reported issue: https://github.com/paritytech/ethabi/issues/40
related #219

external repo with tests: https://github.com/debris/docopt_bug/blob/master/src/lib.rs

tests from external repo run with docopt 0.8.1

running 7 tests
test test_short ... FAILED
test test_short_with_three_params ... FAILED
test test_short_with_one_param ... FAILED
test test_short_with_two_params ... ok
test test_two_shorts_with_three_params ... FAILED
test test_short_with_four_params ... FAILED
test test_two_shorts_with_four_params ... ok

[bjgill]

I think I've run into a related issue in cargo-edit. Our usage string is:

Usage:
    cargo add <crate> [--dev|--build|--optional] [--vers=<ver>|--git=<uri>|--path=<uri>] [options]
    cargo add <crates>... [--dev|--build|--optional] [options]
    cargo add (-h|--help)
    cargo add --version

Specify what crate to add:
    --vers <ver>            Specify the version to grab from the registry (crates.io).
                            You can also specify versions as part of the name, e.g
                            `cargo add [email protected]`.
    --git <uri>             Specify a git repository to download the crate from.
    --path <uri>            Specify the path the crate should be loaded from.

Specify where to add the crate:
    -D --dev                Add crate as development dependency.
    -B --build              Add crate as build dependency.
    --optional              Add as an optional dependency (for use in features). This does not work
                            for `dev-dependencies` or `build-dependencies`.
    --target <target>       Add as dependency to the given target platform. This does not work
                            for `dev-dependencies` or `build-dependencies`.

Options:
    --upgrade=<method>      Choose method of semantic version upgrade. Must be one of
                            "none" (exact version), "patch" (`~` modifier), "minor"
                            (`^` modifier, default), or "all" (`>=`).
    --manifest-path=<path>  Path to the manifest to add a dependency to.
    --allow-prerelease      Include prerelease versions when fetching from crates.io (e.g.
                            '0.6.0-alpha'). Defaults to false.
    -h --help               Show this help page.
    -V --version            Show version.

This command allows you to add a dependency to a Cargo.toml manifest file. If <crate> is a github
or gitlab repository URL, or a local path, `cargo add` will try to automatically get the crate name
and set the appropriate `--git` or `--path` value.

Please note that Cargo treats versions like "1.2.3" as "^1.2.3" (and that "^1.2.3" is specified
as ">=1.2.3 and <2.0.0"). By default, `cargo add` will use this format, as it is the one that the
crates.io registry suggests. One goal of `cargo add` is to prevent you from using wildcard
dependencies (version set to "*").

and the command cargo add failure --vers 0.4.3 --git git://git.git --path /path/here results in the output Args { arg_crate: "", arg_crates: ["failure"], flag_dev: false, flag_build: false, flag_vers: Some("0.4.3"), flag_git: Some("git://git.git"), flag_path: Some("/path/here"), flag_target: None, flag_optional: false, flag_manifest_path: None, flag_version: false, flag_upgrade: None, flag_allow_prerelease: false }.

I believe that that command should be rejected as invalid (as http://try.docopt.org does).

See killercup/cargo-edit#174 for more information