[instagram] Fix extraction after `rhx_gis' field removal

[iamleot]

Instagram has recently removed the `rhx_gis' in sharedData JSON
leading to extraction failures for tags and profiles.

Adjust `X-Instagram-GIS' header generation accordingly.

For completeness here the failure of the tests:

% env PYTHONPATH=. python3.7 test/test_results.py instagram

https://www.instagram.com/p/BqvsDleB3lV/
.
https://www.instagram.com/p/BoHk1haB5tM/
.
https://www.instagram.com/p/Bqxp0VSBgJg/
.
https://www.instagram.com/p/BtOvDOfhvRr/
.
https://www.instagram.com/explore/tags/instagram/
E
https://www.instagram.com/instagram/
E
======================================================================
ERROR: test_InstagramTagExtractor_1 (__main__.TestExtractorResults)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test/test_results.py", line 259, in test
    self._run_test(extr, url, result)
  File "test/test_results.py", line 62, in _run_test
    tjob.run()
  File "test/test_results.py", line 153, in run
    for msg in self.extractor:
  File ".../gallery_dl/extractor/instagram.py", line 32, in items
    for data in self.instagrams():
  File ".../gallery_dl/extractor/instagram.py", line 162, in _extract_tagpage
    yield from self._extract_page(url, 'TagPage')
  File ".../gallery_dl/extractor/instagram.py", line 124, in _extract_page
    rhx_gis = shared_data['rhx_gis']
KeyError: 'rhx_gis'

======================================================================
ERROR: test_InstagramUserExtractor_1 (__main__.TestExtractorResults)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test/test_results.py", line 259, in test
    self._run_test(extr, url, result)
  File "test/test_results.py", line 62, in _run_test
    tjob.run()
  File "test/test_results.py", line 153, in run
    for msg in self.extractor:
  File ".../gallery_dl/extractor/instagram.py", line 32, in items
    for data in self.instagrams():
  File ".../gallery_dl/extractor/instagram.py", line 159, in _extract_profilepage
    yield from self._extract_page(url, 'ProfilePage')
  File ".../gallery_dl/extractor/instagram.py", line 124, in _extract_page
    rhx_gis = shared_data['rhx_gis']
KeyError: 'rhx_gis'

----------------------------------------------------------------------
Ran 6 tests in 5.687s

FAILED (errors=2)

[mikf]

Thank you

[iamleot]

Thanks for quickly applying it!