Use SQLALCHEMY_ENGINE_OPTIONS as default engine options

[ZongXR]

use SQLALCHEMY_ENGINE_OPTIONS as default engine options if engine options is not specified in SQLALCHEMY_BINDS

When engine options is not specified in SQLALCHEMY_BINDS, this engine will lack options with the bind. Which may cause some unexpected errors such as database disconnect error.

SQLALCHEMY_BINDS = {"main": "mysql+pymysql://root:root@localhost:3306/dbname"}
SQLALCHEMY_ENGINE_OPTIONS = {"pool_pre_ping": True}

For example, if someone use Flask-SQLalchemy like this, he will think SQLALCHEMY_ENGINE_OPTIONS will make effect in SQLALCHEMY_BINDS. But that's not the case.

This PR use SQLALCHEMY_ENGINE_OPTIONS as default engine options specifically when engine options is not specified in SQLALCHEMY_BINDS. So the code above will be same as

SQLALCHEMY_BINDS = {
    "main": {
        "url": "mysql+pymysql://root:root@localhost:3306/dbname",
        "pool_pre_ping": True
    }
}

fixes #1396