Add partial support for falsy parameters

Cause falsy permissive comparisons,  all falsy values are considered `''` when computing the signature  

```js
var parameters = { 
 a: '',
 b: null,
 c: undefined,
 d: false,
 e: 0,
 f: [],
 g: true,
 h: 1
};
//...
var signature = oauthSignature.generate(httpMethod, url, parameters, consumerSecret, tokenSecret);

//URL used for signature
//a=&b=&c=&d=&e=&f=&g=true&h=1
```

This PR solves the `false` and `0` use case, following the same approach used for `true` and `Number`, creating an URL 
```js
//a=&b=&c=&d=false&e=0&f=&g=true&h=1
```