Shortcut method has post_body(put_dat) reusing bug.

Curl.get sends last POST request's post_body again. (put_data also)

I wrote simple sinatra application and called GET, POST, and then GET again. 

```ruby
require 'sinatra'

get '/' do
  "Params: #{params.inspect}\nRequest body: #{request.body.string}"
end

post '/' do
  "Params: #{params.inspect}\nRequest body: #{request.body.string}"
end
```


[11] test »  puts Curl.get('http://localhost:4567?a=1&b=2').body_str
Params: {"a"=>"1", "b"=>"2"}
Request body:
[12] test »  puts Curl.post('http://localhost:4567', {:a => 3, :b =>4}).body_str
Params: {"a"=>"3", "b"=>"4"}
Request body: a=3&b=4
=> nil
[13] test »  puts Curl.get('http://localhost:4567?a=1&b=2').body_str
Params: **{"a"=>"3", "b"=>"4"}** **#should be {"a"=>"1", "b"=>"2"}**
Request body: a=3&b=4 ** #should be empty **
=> nil