Make JSORMBase#clearErrors() to through the setter (#77)

We were previously assigning `this._errors` directly, which caused a
problem when reusing reactive frameworks that were observing model
changes by wrapping the existing setters with observers.  Also moved the
clearErrors call into the model's `save()` method out of the payload
writer's asJSON method, which was a very confusing place to have it
live.

Author: wadetandy

src/model.ts

@@ -588,7 +588,7 @@ export class JSORMBase {
   }
 
   clearErrors() {
-    this._errors = {}
+    this.errors = {}
   }
 
   isDirty(relationships?: IncludeScope): boolean {
@@ -820,6 +820,8 @@ export class JSORMBase {
       verb = "patch"
     }
 
+    this.clearErrors()
+
     const json = payload.asJSON()
 
     try {

src/util/write-payload.ts

@@ -133,8 +133,6 @@ export class WritePayload<T extends JSORMBase> {
       type: this.jsonapiType
     }
 
-    this.model.clearErrors()
-
     if (this.model.id) {
       data.id = this.model.id
     }

test/integration/validations.test.ts

@@ -154,6 +154,19 @@ describe("validations", () => {
     expect(instance.errors).to.deep.eq({})
   })
 
+  it("clears errors via the public setter", async () => {
+    fetchMock.restore()
+    fetchMock.mock({
+      matcher: "*",
+      response: { data: { id: "1", type: "employees" } }
+    })
+    let spy = (<any>sinon.spy)(instance, "errors", ["set"])
+
+    await instance.save()
+
+    expect(spy.set).to.have.been.calledOnce
+  })
+
   it("instantiates a new error object instance after save", async () => {
     const originalErrors = (instance.errors = { foo: "bar" })
     const result = instance.save({ with: { books: "genre" } })