fixbug zero prefix id as integer when primary key type is string

the type of model primary key is string, I got 404 error when I get the url:
http://localhost:5000/api/bjlx/01. I found that string '01' convert to 1(int). so it could not found the object for database. 

```
class Bjlx(db.Model):
    bjlxdm = db.Column(db.String(2), primary_key=True)
    bjlxmc = db.Column(db.String(64), nullable=False)

```