header type check bug

In `electron@5.x`
I find an interesting bug.



I set headers with object, but the object type gave `Map`, so the header think it's not an object
```
header: function (field, value) {
if (is(field).a(Object)) {
for (var key in field) {
if (Object.prototype.hasOwnProperty.call(field, key)) {
$this.header(key, field[key])
}
}

return $this
}

var existingHeaderName = $this.hasHeader(field)
$this.options.headers[existingHeaderName || field] = value

return $this
},
```

see the error stack:
```
name.toLowerCase is not a function
at Object.hasHeader (/Users/xiwz/ynote/electron-sdk/example/node_modules/unirest/index.js:93:21)
at Object.header (/Users/xiwz/ynote/electron-sdk/example/node_modules/unirest/index.js:198:40)
```

But actually I send the object
```
{
'Content-Type': '',
'Cookie': ''
}
```

I think this bug belongs to electron.It changed the Object default behavior. But `unirest` type check is unsafe.

I wonder if you would like to improve it ? I am willing to send a PR.

My solution:
```
if (check.prototype) check = Object.prototype.toString.call(check.prototype).slice(8, -1).toLowerCase()
```

Inspect the header and hasHeader entry points shown in unirest/index.js at lines 93 and 198, then reproduce the object-header case under electron@5.x. Done means object headers such as Content-Type and Cookie no longer cause the name.toLowerCase error.

Written by the indexing model from the issue text.