Fix decoding non utf-8 characters.

[jgoclawski]

The API does not assume any particular encoding, so there are no sane defaults. Strings can be encoded with various encodings and are transferred as bytes. For one user latin-1 will be the best one to use to decode those bytes, for another - windows-1250 will work better. Let's use utf-8 and escape unkown bytes.
Ref. https://forum.mikrotik.com/viewtopic.php?t=106053#p528460

It is possible to use specific encoding by defining custom default structure like this:

import collections
import routeros_api
from routeros_api.api_structure import StringField

connection = routeros_api.RouterOsApiPool('ip', username='admin', password='password', plaintext_login=True)
api = connection.get_api()
# This part here is important:
default_structure = collections.defaultdict(lambda: StringField(encoding='windows-1250'))
api.get_resource('/system/identity', structure=default_structure).get()

Fixes: #53
Fixes: #68

Supersedes: #58
Supersedes: #69
Supersedes: #74
Supersedes: #82
Supersedes: #90
Supersedes: #101

[levonyan021]

Thank you