Fix bug displaying nil for relationship link href

When using the relationship link with a block, calling "meta" without "href", e.g.
has_one :bio do
  link :related do
    meta id: 1
  end
end

results in in a nil "href", e.g.
{ links: { posts: { related: { href: nil, meta: { id: 1 } }  } } }.
According to JSONAPI, we should be able to use meta without href
(http://jsonapi.org/format/#document-links).

lib/active_model/serializer/adapter/json_api/link.rb

@@ -28,8 +28,9 @@ module ActiveModel
           def as_json
             return @value if @value
 
-            hash = { href: @href }
-            hash.merge!(meta: @meta) if @meta
+            hash = {}
+            hash[:href] = @href if @href
+            hash[:meta] = @meta if @meta
 
             hash
           end