Fix mysql return bytes as field name type

[sdvcrx]

This fix different behavior of mysql-connector-python C extension and pure python version.

Fix #758 #766

How to repeat:

Install mysql, pyspider, then install mysql-connector-python via pip :

pip install mysql-connector-python  # current version 8.0.11

Run python script:

from pyspider.database.mysql.taskdb import TaskDB

db = TaskDB(user='root', database='db', passwd='password')
print(db.get_task('projectname', 'taskid'))

and output:

{'taskid': 'taskid',
 'project': 'project',
 'url': 'url',
 'status': 1,
 b'schedule': '{"itag": "v2", "age": 86400}',    // <- b'schedule' but expect 'schedule'
 b'fetch': None,
 b'process': None,
 b'track': None,
 'lastcrawltime': None,
 'updatetime': None}

Related: mysql/mysql-connector-python#37

[binux]

Thank you very much!