Update issue.rb to not escape &fields query parameter

Don't escape &fields part of the URL when specifying fields to return in a jql Issue query.

When I was issuing requests like this:
issues = client.Issue.jql(request, ["summary"])
I was getting this error:
C:/Ruby193/lib/ruby/gems/1.9.1/gems/jira-ruby-0.1.10/lib/jira/request_client.rb:14:in `request': Bad Request (JIRA::HTTPError)

This change resolves that problem.