fix: xhr.withCredentials is initially `false`

According to the [XMLHttpRequest2 spec](http://www.w3.org/TR/XMLHttpRequest2/#the-withcredentials-attribute),
`xhr.withCredentials` should be initially `false`. http-browserify sets this
flag to `true` by default which disobeys the spec. This leads to browser errors when
making CORS requests to domains that have wildcards in their
Access-Control-Allow-Origin header.

http-browserify should attempt to follow the spec by default. In this case,
that means setting `withCredentails` to `false` initially, and then allowing
the user to override that in the passing in `params`.

Maybe it is possible to auto-detect when [user credentials](http://www.w3.org/TR/XMLHttpRequest2/#user-credentials)
are being sent and then set the `withCredentials` flag from there.

Somewhat related to #35 (the committer there expressed concern about
`withCredentials` being `true` when unintialzed as well).