Allow fields option to be passed with string keys.

lib/jsonapi/renderer.rb

@@ -7,13 +7,14 @@ module JSONAPI
   #
   # @param [Hash] params
   #   @option [(#jsonapi_id, #jsonapi_type, #jsonapi_related, #as_jsonapi),
-  #           Array<(#jsonapi_id, #jsonapi_type, #jsonapi_related, #as_jsonapi)>,
+  #           Array<(#jsonapi_id, #jsonapi_type, #jsonapi_related,
+  #           #as_jsonapi)>,
   #           nil] data Primary resource(s) to be rendered.
   #   @option [Array<#jsonapi_id>] errors Errors to be rendered.
-  #   @option [String, Hash{Symbol => Hash}] include Relationships to be
-  #     included.
-  #   @option [Hash{Symbol, Array<Symbol>}] fields List of requested fields
-  #     for some or all of the resource types.
+  #   @option include Relationships to be included. See
+  #     JSONAPI::IncludeDirective.
+  #   @option [Hash{Symbol, Array<Symbol>}, Hash{String, Array<String>}] fields
+  #     List of requested fields for some or all of the resource types.
   #   @option [Hash] meta Non-standard top-level meta information to be
   #     included.
   #   @option [Hash] links Top-level links to be included.

lib/jsonapi/renderer/document.rb

@@ -9,7 +9,7 @@ def initialize(params = {})
         @errors  = params.fetch(:errors,  [])
         @meta    = params.fetch(:meta,    nil)
         @links   = params.fetch(:links,   {})
-        @fields  = params.fetch(:fields,  {})
+        @fields  = _symbolize_fields(params.fetch(:fields, {}))
         @jsonapi = params.fetch(:jsonapi, nil)
         @include = JSONAPI::IncludeDirective.new(params.fetch(:include, {}))
       end
@@ -48,6 +48,12 @@ def errors_hash
           hash[:errors] = @errors.map(&:as_jsonapi)
         end
       end
+
+      def _symbolize_fields(fields)
+        fields.each_with_object({}) do |(k, v), h|
+          h[k.to_sym] = v.map(&:to_sym)
+        end
+      end
     end
   end
 end