Add OxfordDictionary::Endpoints::Thesaurus

Oxford Dictionaries is updating their API to a new version which
includes quite a few changes:
https://developer.oxforddictionaries.com/version2

They are moving the thesaurus functionality to its own endpoint
(instead of having part of the entries endpoint).

As mentioned in the tests, this only unit tests the endpoint. If someone
would like to contribute, please feel free to update and run the specs
(similar to what was done for the Entries and Lemmas endpoints).

Entries: #8
Lemmas: #10

lib/oxford_dictionary/endpoints/thesaurus.rb

@@ -0,0 +1,21 @@
+require 'oxford_dictionary/endpoints/endpoint'
+
+module OxfordDictionary
+  module Endpoints
+    class Thesaurus < Endpoint
+      ENDPOINT = 'thesaurus'.freeze
+
+      def thesaurus(word:, language:, params: {})
+        query_string = "#{ENDPOINT}/#{language}/#{word}"
+        uri = URI(query_string)
+
+        unless params.empty?
+          uri.query = URI.encode_www_form(params)
+        end
+
+        response = @request_client.get(uri: uri)
+        deserialize.call(response.body)
+      end
+    end
+  end
+end

spec/endpoints/thesaurus_spec.rb

@@ -0,0 +1,48 @@
+require 'spec_helper'
+require 'oxford_dictionary/endpoints/thesaurus'
+
+# Spec dependencies
+require 'oxford_dictionary/request'
+
+RSpec.describe OxfordDictionary::Endpoints::Thesaurus do
+  let(:request_client) do
+    OxfordDictionary::Request.
+      new(app_id: ENV['APP_ID'], app_key: ENV['APP_KEY'])
+  end
+
+  let(:endpoint) { described_class.new(request_client: request_client) }
+
+  let(:word) { 'ace' }
+  let(:language) { 'en' }
+  let(:params) { { fields: 'synonyms,antonyms' } }
+
+  # The sentences endpoint is only avaiable to the paid tier
+  # If someone with a paid tier account would like to contribute, please
+  # feel free remove this double (and the stub in the tests), uncomment the
+  # sections that run VCR against the live endpoint, and PR the resulting files
+  let(:response_double) { double(body: {}.to_json) }
+
+  describe '#thesaurus' do
+    subject do
+      endpoint.thesaurus(word: word, language: language, params: params)
+    end
+
+    it 'calls API as expected', :aggregate_failures do
+      expected_uri =
+        URI("thesaurus/#{language}/#{word}?fields=synonyms%2Cantonyms")
+
+      expect(request_client).to receive(:get).
+        with(uri: expected_uri).
+        and_return(response_double)
+
+      subject
+
+      # VCR.use_cassette('sentences#sentence') do
+      #   response = subject
+      #   expect(response).to be_an(OpenStruct)
+      #   expect(response.results.first.id).to eq(word)
+      #   expect(response.results.first.lexicalEntries).to all(be_an(OpenStruct))
+      # end
+    end
+  end
+end