do not use http_proxy environment for Net::HTTP (default from ruby 2.0)

http://ruby-doc.org/stdlib-2.0.0/libdoc/net/http/rdoc/Net/HTTP.html#method-c-new

starting from ruby 2.0 Net::HTTP.new('host', 'port') checks for http_proxy ENV and automatically uses the proxy if so.

this patch creates Net::HTTP with no proxy if it is not passed into SplunkClient.new