Add support for dynamic string-links in JsonApi adapter.

lib/active_model/serializer/adapter/json_api.rb

@@ -210,15 +210,7 @@ module ActiveModel
 
         def links_for(serializer)
           serializer._links.each_with_object({}) do |(name, value), hash|
-            hash[name] =
-              if value.respond_to?(:call)
-                link = Link.new(serializer)
-                link.instance_eval(&value)
-
-                link.to_hash
-              else
-                value
-              end
+            hash[name] = Link.new(serializer, value).as_json
           end
         end
 

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

@@ -3,29 +3,39 @@ module ActiveModel
     module Adapter
       class JsonApi
         class Link
-          def initialize(serializer)
+          def initialize(serializer, value)
             @object = serializer.object
             @scope = serializer.scope
+
+            # Use the return value of the block unless it is nil.
+            if value.respond_to?(:call)
+              @value = instance_eval(&value)
+            else
+              @value = value
+            end
           end
 
           def href(value)
-            self._href = value
+            @href = value
+            nil
           end
 
           def meta(value)
-            self._meta = value
+            @meta = value
+            nil
           end
 
-          def to_hash
-            hash = { href: _href }
-            hash.merge!(meta: _meta) if _meta
+          def as_json
+            return @value if @value
+
+            hash = { href: @href }
+            hash.merge!(meta: @meta) if @meta
 
             hash
           end
 
           protected
 
-          attr_accessor :_href, :_meta
           attr_reader :object, :scope
         end
       end