Fix compatibility issue when converting a Windows path to a URL.

On Windows, the output of the following code is ['/static/foo\bar'], which works fine on Linux as ['/static/foo/bar']. The issue arises from the difference between Windows' path separator (\\) and the URL separator (/), well on Linux, path separator is same as the URL separator.

from flask import Flask
from flask_assets import Environment, Bundle

app = Flask(__name__)
env = Environment(app)
print(Bundle("foo/bar", env=env).urls())