fixing browser test

looks like an update to express broke the "Content-Type params" test because
express doesn't seem to append the charset=utf-8 in cases of res.json() or
where the content-type is explicitly set

also updated the url on the readme to point to port 4000, which is what
`make test-server` actually uses
Merge pull request #253 from dominicbarnes/browser-test-fix

Quick Browser Test Fix
add browser field to support browserify

Allows for superagent to be used with browserify (or other browser
field compatible packaging tools).
Merge pull request #259 from shtylman/browser-field

add browser field to support browserify