Add Sparse Fieldset support for Attributes

Sources/JSONAPI/SparseFields/SparseFieldEncodable.swift

@@ -0,0 +1,31 @@
+//
+//  SparseField.swift
+//  
+//
+//  Created by Mathew Polzin on 8/4/19.
+//
+
+public struct SparseField<
+    Description: JSONAPI.ResourceObjectDescription,
+    MetaType: JSONAPI.Meta,
+    LinksType: JSONAPI.Links,
+    EntityRawIdType: JSONAPI.MaybeRawId
+>: Encodable where Description.Attributes: SparsableAttributes {
+
+    public typealias Resource = JSONAPI.ResourceObject<Description, MetaType, LinksType, EntityRawIdType>
+
+    public let resourceObject: Resource
+    public let fields: [Description.Attributes.CodingKeys]
+
+    public init(_ resourceObject: Resource, fields: [Description.Attributes.CodingKeys]) {
+        self.resourceObject = resourceObject
+        self.fields = fields
+    }
+
+    public func encode(to encoder: Encoder) throws {
+        let sparseEncoder = SparseFieldEncoder(wrapping: encoder,
+                                               encoding: fields)
+
+        try resourceObject.encode(to: sparseEncoder)
+    }
+}