Don't attempt to follow redirection w/o Location.

When there is no Location header, we cannot follow any redirection.

Per RFC 7231, a server SHOULD generate a Location header field in the
response for redirection requests, but servers do not always do this.

https://tools.ietf.org/html/rfc7231#section-6.4.3

Fixes: #481

lib/restclient/abstract_response.rb

@@ -167,6 +167,11 @@ module RestClient
       # parse location header and merge into existing URL
       url = headers[:location]
 
+      # cannot follow redirection if there is no location header
+      unless url
+        raise exception_with_response
+      end
+
       # handle relative redirects
       unless url.start_with?('http')
         url = URI.parse(request.url).merge(url).to_s

spec/unit/abstract_response_spec.rb

@@ -104,5 +104,12 @@ describe RestClient::AbstractResponse, :include_helpers do
       @response.should_receive(:follow_redirection).and_return('fake-redirection')
       @response.return!.should eq 'fake-redirection'
     end
+
+    it "should gracefully handle 302 redirect with no location header" do
+    @net_http_res = response_double(code: 302, location: nil)
+    @request = request_double()
+    @response = MyAbstractResponse.new(@net_http_res, @request)
+      lambda { @response.return! }.should raise_error RestClient::Found
+    end
   end
 end