Improve HTTPClient versions compatibility.

httpclient changed Message#body method as an alias of Message#content
from 2.2.0. This change keeps both httpclient 2.2.0 and < 2.2.0
compatibility.

lib/webmock/http_lib_adapters/httpclient.rb

@@ -119,7 +119,7 @@ def build_request_signature(req)
     request_signature = WebMock::RequestSignature.new(
       req.header.request_method.downcase.to_sym,
       uri.to_s,
-      :body => req.body.content,
+      :body => req.content,
       :headers => headers
     )
   end