Fix spaces encoding in parameters

OAuth requires spaces to be encoded as %20 instead of + in order to generate a valid signature.

Explained in http://troy.yort.com/2-common-problems-with-oauth-client-libraries/

Before this patch, I was unable to use Yahoo!'s BOSS Placefinder API, which now seems to be responding correctly to me, instead of what it used to say:

```
<?xml version='1.0' encoding='UTF-8'?>
<yahoo:error xmlns:yahoo='http://yahooapis.com/v1/base.rng'
  xml:lang='en-US'>
  <yahoo:description>Please provide valid credentials. OAuth oauth_problem="signature_invalid", realm="yahooapis.com"</yahoo:description>
</yahoo:error>
```