QueryBuilder with model specified does not default to using the model's table

[elgow]

Describe the feature as you'd like to see it
When a model= is specified in the constructor for a QueryBuilder and no table is explicitly specified by table() then the builder should default to using the model's table.

What do we currently have to do now?
In the current code it is necessary to redundantly specify the table or it defaults to None. This causes exceptions and causes join() etc. to produce broken SQL.

class User(Model):
    __timestamps__ = False
    
QueryBuilder(model=User).table('users').get_table_name()
Out[20]: 'users'

QueryBuilder(model=User).get_table_name()
Traceback (most recent call last):
 ...
 line 228, in get_table_name
    return self._table.name
AttributeError: 'NoneType' object has no attribute 'name'

QueryBuilder(model=User).join('bar').to_sql()
Out[22]: 'SELECT *'

Additional context
When the model is given it is more reasonable to expect that its table will be used. This commit proposes code to fix.

b3f9bca

• Is this a breaking change?
  No. Explicit table() could still override