fix base64 encoded string

[KazuCocoa]

Fix: function find_element_by_image always raise an exception: "TypeError: Object of type 'bytes' is not JSON serializable" #228

encodestring generate byte data in Python 3. Thus, we should encode it to string.

Python2.7

>>> import base64
>>> base64.encodestring(b'hello')
'aGVsbG8=\n'

Python3.6

>>> import base64
>>> base64.encodestring(b'hello')
b'aGVsbG8=\n'

b64_data should be base64 encoded string.
This way is as same as https://github.com/SeleniumHQ/selenium/blob/6b5fe890da7d1592693d1dfcf795d882fc0426c5/py/selenium/webdriver/firefox/firefox_profile.py#L180 way.

[KazuCocoa]

I could call json.dump(b64_data) without exception in Python2/3

with open('/Users/kazu/GitHub/python-client/sample.JPG', 'rb') as f:
    b64_data = base64. b64encode(f.read()).decode('UTF-8')

json.dumps(b64_data)