Values with spaces are encoded incorrectly

[okainov]

If one passes ["mydata" => "data with spaces"] then one will get "Invalid signature" error, because even if in the request there were spaces, https://github.com/ofmlabs/oauth-php/blob/master/library/OAuthRequest.php#L751 this piece of code gets all spaces as + (and + as %2B). Easy fix is to replace + => %20 there