Fix readme merge example by using Hash#update

Squashes the following:

commit 0545399
Author: Ismael Celis <[email protected]>
Date:   Thu May 1 16:18:05 2014 +0100

    Hash#update metter than #merge!

commit d960813
Author: Ismael Celis <[email protected]>
Date:   Thu May 1 16:15:33 2014 +0100

    Fix readme merge example

Author: ismasan

Readme.md

@@ -16,7 +16,7 @@ Example API client:
 ```ruby
 api = Hyperclient.new('http://myapp.com/api').tap do |api|
   api.digest_auth('user', 'password')
-  api.headers.merge({'accept-encoding' => 'deflate, gzip'})
+  api.headers.update({'accept-encoding' => 'deflate, gzip'})
 end
 ```