Flask: Add context injection safeties

[pglombardo]

This fixes the case of:

During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/instana/instrumentation/flask/with_blinker.py", line 105, in handle_user_exception_with_instana
    response.headers.add('Server-Timing', "intid;desc=%s" % scope.span.context.trace_id)
AttributeError: 'dict' object has no attribute 'add'