Make offset and paged paginators links consistent

Per the pagination spec (http://jsonapi.org/format/#fetching-pagination)
previous links should be 'prev'.  It was done this way in the paged
paginator but not the offset.

Author: coryodaniel

lib/jsonapi/paginator.rb

@@ -58,10 +58,12 @@ def links_page_params(options = {})
 
       previous_offset = 0 if previous_offset < 0
 
-      links_page_params['previous'] = {
+      links_page_params['prev'] = {
         'offset' => previous_offset,
         'limit' => @limit
       }
+
+      links_page_params['previous'] = links_page_params['prev']
     end
 
     next_offset = @offset + @limit

test/controllers/controller_test.rb

@@ -3053,7 +3053,7 @@ def test_books_operation_links
     assert_response :success
     assert_equal 12, json_response['data'].size
     assert_equal 'Book 50', json_response['data'][0]['attributes']['title']
-    assert_equal 5, json_response['links'].size
+    assert_equal 6, json_response['links'].size
     assert_equal 'https://test_corp.com', json_response['links']['spec']
   ensure
     JSONAPI.configuration = original_config

test/unit/pagination/offset_paginator_test.rb

@@ -156,7 +156,7 @@ def test_offset_links_page_params_large_data_set_before_start
     paginator = OffsetPaginator.new(params)
     links_params = paginator.links_page_params(record_count: 50)
 
-    assert_equal 4, links_params.size
+    assert_equal 5, links_params.size
 
     assert_equal 5, links_params['first']['limit']
     assert_equal 0, links_params['first']['offset']
@@ -182,7 +182,7 @@ def test_offset_links_page_params_large_data_set_middle
     paginator = OffsetPaginator.new(params)
     links_params = paginator.links_page_params(record_count: 50)
 
-    assert_equal 4, links_params.size
+    assert_equal 5, links_params.size
 
     assert_equal 5, links_params['first']['limit']
     assert_equal 0, links_params['first']['offset']
@@ -208,7 +208,7 @@ def test_offset_links_page_params_large_data_set_end
     paginator = OffsetPaginator.new(params)
     links_params = paginator.links_page_params(record_count: 50)
 
-    assert_equal 3, links_params.size
+    assert_equal 4, links_params.size
 
     assert_equal 5, links_params['first']['limit']
     assert_equal 0, links_params['first']['offset']
@@ -231,7 +231,7 @@ def test_offset_links_page_params_large_data_set_past_end
     paginator = OffsetPaginator.new(params)
     links_params = paginator.links_page_params(record_count: 50)
 
-    assert_equal 3, links_params.size
+    assert_equal 4, links_params.size
 
     assert_equal 5, links_params['first']['limit']
     assert_equal 0, links_params['first']['offset']