Added test coverage to mw-api-version-mismatch

refs https://github.com/TryGhost/Toolbox/issues/292

- There's no good reason to not write tests!

ghost/mw-api-version-mismatch/test/hello.test.js

@@ -1,8 +0,0 @@
-const assert = require('assert');
-
-describe('Hello world', function () {
-    it('Runs a test', function () {
-        // TODO: Write me!
-        assert.equal('hello', 'hello');
-    });
-});

ghost/mw-api-version-mismatch/test/mw-api-version-mismatch.test.js

@@ -0,0 +1,37 @@
+const assert = require('assert');
+const sinon = require('sinon');
+const errors = require('@tryghost/errors');
+
+const versionMismatchMW = require('../index');
+
+describe('mw-api-version-mismatch', function () {
+    it('Does call handle mismatch when a generic RequestNotAcceptableError is used', function (done) {
+        const APIVersionCompatibilityService = {
+            handleMismatch: sinon.stub().resolves()
+        };
+        const req = {
+            headers: {}
+        };
+        const res = {
+            locals: {}
+        };
+
+        versionMismatchMW(APIVersionCompatibilityService)(new errors.RequestNotAcceptableError({
+            code: 'UPDATE_CLIENT'
+        }), req, res, () => {
+            assert.equal(APIVersionCompatibilityService.handleMismatch.called, true);
+            done();
+        });
+    });
+
+    it('Does NOT call handle mismatch when a generic RequestNotAcceptableError is used', function (done) {
+        const APIVersionCompatibilityService = {
+            handleMismatch: sinon.stub().resolves()
+        };
+
+        versionMismatchMW(APIVersionCompatibilityService)(new errors.RequestNotAcceptableError(), {}, {}, () => {
+            assert.equal(APIVersionCompatibilityService.handleMismatch.called, false);
+            done();
+        });
+    });
+});