Avoid undefined values on query parameters.

From an object like:
```js
var params = {
  hello: 'world',
  bye: undefined
```

the result should only be `hello=world&bye=undefined` if `bye: 'undefined'`
add test and update build
Avoid null values on query parameters
Update build and tests for client lib
Merge branch 'patch-1' of git://github.com/cristiandouce/superagent into cristiandouce-patch-1

* 'patch-1' of git://github.com/cristiandouce/superagent:
  Update build and tests for client lib
  Avoid null values on query parameters
  add test and update build
  Avoid undefined values on query parameters.