Add partial support for falsy parameters

[jmendiara]

Cause falsy permissive comparisons, all falsy values are considered '' when computing the signature

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

//a=&b=&c=&d=false&e=0&f=&g=true&h=1

[jmendiara]

Sorry, I have no time for making test

[bettiolo]

Hi @jmendiara,
The comment I wrote for the issue #10 answers this as well:
#10 (comment)

Please, tell me if this clarifies the situation.

[jmendiara]

This is a matter of parameter serialization, not RFC stuff. If I want to send
GET http://foo.com/bar?from=0&to=1&oauthStuff

I would need to encode the parameters this way

var params = {
   from: '0',
   to: 1
};
oauthSignature.generate(..., params, ....);

otherwise, the server signature will fail cause of you signed the url
GET http://foo.com/bar?from=&to=1&oauthStuff