Use Rack::Utils instead of Addressable to build query

Previously, the query was constrcuted by using
Addressable's query_values method. Up until version 2.2.8,
this allowed setting nested hashes. From 2.3 onward, nested
hashes are not supported. In order to upgrade Addressable,
this replaces the query handling with Rack::Utils.build_query.
Bump version to 1.3.9

The recent changes concerning query building should be
fully backwards compatible.