unknown "--option=value notation" value consumed as defaultOption

This output from this example is incorrect.

```js
> const commandLineArgs = require('command-line-args')
> const definitions = [ { name: 'file', type: String, defaultOption: true } ]
> commandLineArgs(definitions, { partial: true, argv: [ '--two=3' ] })
{ file: '3', _unknown: [ '--two' ] }
```
It should result in this output:

```js
{ _unknown: [ '--two=3' ] }
```
