Insert operations of models with non-PK foreign keys fail when passing a target object instance

[smuething]

I have a model A with a ForeignKeyField column that uses the field parameter to set a non-pk reference column in the target model:

class Target(Model):
    key = FixedCharField(8,unique=True)

class A(Model):
    target = ForeignKeyField(Target,field="key",lazy_load=False)

When trying to create instances of A, I can do so with create(), but not with the ModelInsert-based methods:

t = Target.create(key="X")

A.create(target=t) # works

A.insert(target="X").execute() # works
A.insert(target=t).execute()   # fails

In the last case, the generated SQL uses the primary key of t instead of t.key because the code path in ModelInsert does not apply the field parameter of the ForeignKeyField to the passed-in model instance. The attached patch fixes the problem for me.