text: Title: factory.build() saves instance to database
  Body: #### Description
  My understanding from the documentation (https://factoryboy.readthedocs.io/en/latest/index.html?highlight=build#using-factories) is that the build strategy will not save the instance to the database.
  
  ```WidgetFactory.build()``` does not save the instance to the database, but ```factory.build(WidgetFactory)``` does.
  
  #### To Reproduce
  Using Python 3.7.2, Django 2.1.7, faker 1.0.2, and factory-boy 2.11.1
  
  ##### Model / Factory code
  ```python
  # models.py
  from django.db import models
  
  class Widget(models.Model):
      name = models.CharField(max_length=25)
  
  # tests/factories.py
  from factory import DjangoModelFactory, lazy_attribute
  from faker import Faker
  from ..models import Widget
  
  faker = Faker()
  
  class WidgetFactory(DjangoModelFactory):
      class Meta:
          model = Widget
  
      name = lazy_attribute(lambda x: faker.word())
  
  ```
  
  ##### The issue
  Here are the steps to reproduce in the REPL:
  
  ```python
  >>> import factory
  >>> from widgets.tests.factories import WidgetFactory
  >>> w1 = WidgetFactory.create()
  >>> w1.pk
  4
  >>> w2 = WidgetFactory.build()
  >>> w2.pk
  # (w2.pk is None, as expected)
  >>> w3 = factory.create(WidgetFactory)
  >>> w3.pk
  5
  >>> w4 = factory.build(WidgetFactory)
  >>> w4.pk
  6 # (was not expecting the instance to be saved)
  ```
  
  #### Notes
  Perhaps I've misunderstood something here.  Should ```WidgetFactory.build()``` and ```factory.build(WidgetFactory)``` have the same behavior?
  