B703 and B308 check passed if mark_safe used as a decorator

Describe the bug
According to docs mark_safe can also be used as a decorator.

B703 and B308 checks in this case passed.

To Reproduce

When mark_safe used as a function:

example.py

from django.utils.safestring import mark_safe


def foo(a):
    return mark_safe(a)

Output

bandit example.py 
[main]	INFO	profile include tests: None
[main]	INFO	profile exclude tests: None
[main]	INFO	cli include tests: None
[main]	INFO	cli exclude tests: None
[main]	INFO	running on Python 3.7.5
[node_visitor]	INFO	Unable to find qualified name for module: example.py
Run started:2020-03-25 14:34:46.110414

Test results:
>> Issue: [B703:django_mark_safe] Potential XSS on mark_safe function.
   Severity: Medium   Confidence: High
   Location: example.py:5
   More Info: https://bandit.readthedocs.io/en/latest/plugins/b703_django_mark_safe.html
4	def foo(a):
5	    return mark_safe(a)

--------------------------------------------------
>> Issue: [B308:blacklist] Use of mark_safe() may expose cross-site scripting vulnerabilities and should be reviewed.
   Severity: Medium   Confidence: High
   Location: example.py:5
   More Info: https://bandit.readthedocs.io/en/latest/blacklists/blacklist_calls.html#b308-mark-safe
4	def foo(a):
5	    return mark_safe(a)

--------------------------------------------------

Code scanned:
	Total lines of code: 3
	Total lines skipped (#nosec): 0

Run metrics:
	Total issues (by severity):
		Undefined: 0.0
		Low: 0.0
		Medium: 2.0
		High: 0.0
	Total issues (by confidence):
		Undefined: 0.0
		Low: 0.0
		Medium: 0.0
		High: 2.0
Files skipped (0):

When as a decorator

example.py

from django.utils.safestring import mark_safe

@mark_safe
def foo(a):
    return a

Output

bandit example.py 
[main]	INFO	profile include tests: None
[main]	INFO	profile exclude tests: None
[main]	INFO	cli include tests: None
[main]	INFO	cli exclude tests: None
[main]	INFO	running on Python 3.7.5
[node_visitor]	INFO	Unable to find qualified name for module: example.py
Run started:2020-03-25 14:35:07.494061

Test results:
	No issues identified.

Code scanned:
	Total lines of code: 4
	Total lines skipped (#nosec): 0

Run metrics:
	Total issues (by severity):
		Undefined: 0.0
		Low: 0.0
		Medium: 0.0
		High: 0.0
	Total issues (by confidence):
		Undefined: 0.0
		Low: 0.0
		Medium: 0.0
		High: 0.0
Files skipped (0):

Expected behavior
Same issues should be shown.

Bandit version

bandit 1.6.2
  python version = 3.7.6 (default, Jan 18 2020, 02:49:59) [GCC 9.2.0]

Additional context
I like this tool :)

Start by locating the B703 and B308 check implementations and their existing tests. Reproduce the reported @mark_safe example alongside the function-call example, then run the relevant Bandit test suite. Done means the decorator form receives the same findings as the direct call without regressing existing checks.