get_url returns different urls calling twice (bug #506)

[ehooo]

• Describe the bug
  The function get_url overwrite the info['id'] stored on extension_loader.MANAGER.blacklist_by_id

• Steps to reproduce the behavior:

In [1]: from bandit.core.docs_utils import get_url                                                                                                                                                                                                                      
In [2]: get_url('B304')                                                                                                                                                                                                                                                 
Out[2]: 'https://bandit.readthedocs.io/en/latest/blacklists/blacklist_calls.html#b304-b305-ciphers-and-modes'
In [3]: get_url('B304')                                                                                                                                                                                                                                                 
Out[3]: 'https://bandit.readthedocs.io/en/latest/blacklists/blacklist_imports.html#b304-b305-ciphers-and-modes'

• Expected behavior
  Call to bid with same bid must return same url

[ericwb]

Please also add a better description in your PR message and title.

[ericwb]

Looks good