IDGenerator support for xxxHistory SObjects

[cropredyHelix]

Currently, fflib_IDGenerator.generate(ContactHistory.SObjectType) returns null000000000000 for any xxxHistory object because getDescribe().getKeyPrefix() returns null for these SObjects

Extend the class's generate method to look like this:

 public static Id generate(Schema.SObjectType sobjectType) {
   String keyPrefix = sobjectType.getDescribe().getKeyPrefix();
   if (keyPrefix == null) {
    keyPrefix =  sobjectType.getDescribe().getName().endsWith('History') ? '017' : null;
   }

   fakeIdCount++;
   String fakeIdPrefix = ID_PATTERN.substring(0, 12 - fakeIdCount.format().length());
   return Id.valueOf(keyPrefix + fakeIdPrefix + fakeIdCount);
 }

While mocking xxxHistory records is problematic as you can't mock OldValue or NewValue fields including via Json.deserialize, you can mock the other fields (Id, ParentId, Field) via Json.deserialize and hence can produce selector mocks for use cases such as testing Field History deletion (introduced in V42)

[joeythomaschaske]

This issue also affects __Share records