add read_timeout support for requests

Hello, i would like to be able to define the read_timeout in my code. I found this issue https://github.com/ruby-rdf/sparql-client/pull/33 .

I have copied the commit https://github.com/ncbo/sparql-client/commit/39149817eda009006ab030c3f8d58daaaec811ea comming from palexander.

Thanks for your work, chris