Display type correctly for array if obj passed as an array (#1394)

* Display type correctly for array if obj passed as an array

* Narrow the test case for `should throw an error if data is an array`

Author: ilyasakin

__tests__/server/validate-data.js

@@ -7,7 +7,7 @@ describe('validateData', () => {
   })
 
   test('should throw an error if data is an array', () => {
-    assert.throws(() => validateData([]), /must be an object/)
+    assert.throws(() => validateData([]), /Data must be an object. Found array./)
   })
 
   test("shouldn't throw an error", () => {

src/server/router/validate-data.js

@@ -17,7 +17,7 @@ module.exports = (obj) => {
     Object.keys(obj).forEach(validateKey)
   } else {
     throw new Error(
-      `Data must be an object. Found ${typeof obj}.` +
+      `Data must be an object. Found ${Array.isArray(obj) ? 'array' : typeof obj}.` +
         'See https://github.com/typicode/json-server for example.'
     )
   }