chore(iast): fix header injection attr (#13624)

Fix validation introduced in
#13546

APPSEC-57163

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Author: avara1986

ddtrace/appsec/_iast/_logs.py

@@ -49,4 +49,4 @@ def iast_error(msg, default_prefix="iast::"):
         stack = inspect.stack()
         frame_info = "\n".join("%s %s" % (frame_info.filename, frame_info.lineno) for frame_info in stack[:7])
         log.debug("%s%s:\n%s", default_prefix, msg, frame_info)
-        _set_iast_error_metric(f"{default_prefix}. {msg}")
+        _set_iast_error_metric(f"{default_prefix}{msg}")

ddtrace/appsec/_iast/taint_sinks/header_injection.py

@@ -207,12 +207,10 @@ class HeaderInjection(VulnerabilityBase):
 
 def _iast_django_response(wrapped, instance, args, kwargs):
     try:
-        from django import VERSION as DJANGO_VERSION
-
         wrapped.__func__(instance, *args, **kwargs)
-        if DJANGO_VERSION < (3, 2, 0):
+        if hasattr(instance, "_headers"):
             instance._headers = HeaderInjectionDict(instance._headers)
-        else:
+        elif hasattr(instance, "_store"):
             instance._store = HeaderInjectionDict(instance._store)
     except Exception as e:
         iast_error(f"propagation::sink_point::Error in _iast_django_response. {e}")