Fixes discover.movie and discover.tv kwargs iteration bug.

[carlos-avila]

It's necessary to create a copy of kwargs before modifying it. It wasn't processing the arguments properly and was returning unpredictable results.

[celiao]

Hey @carlos-avila, can you send me an example to demonstrate how the original code wasn't working properly, and how with your fix, it is?

[celiao]

Ah. The iteration bug appears to surface when Python 3.8 is used.

Before the suggested change:

# Virtual Environment with Python 3.7.7
(tmdb-3.7.7) % python -m unittest tests.test_discover
...
----------------------------------------------------------------------
Ran 3 tests in 3.694s

OK

# Virtual Environment with Python 3.8.3
(tmdb-3.8.3) % python -m unittest tests.test_discover
..E
======================================================================
ERROR: test_discover_tv_underscore (tests.test_discover.DiscoverTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/celia/python/tmdbsimple/tests/test_discover.py", line 44, in test_discover_tv_underscore
    response = discover.tv(page=2,
  File "/Users/celia/python/tmdbsimple/tmdbsimple/discover.py", line 170, in tv
    for param in kwargs:
RuntimeError: dictionary keys changed during iteration

----------------------------------------------------------------------
Ran 3 tests in 1.838s

FAILED (errors=1)

After the suggested change:

# Virtual Environment with Python 3.7.7
(tmdb-3.7.7) % python -m unittest tests.test_discover
...
----------------------------------------------------------------------
Ran 3 tests in 3.694s

OK

# Virtual Environment with Python 3.8.3
(tmdb-3.8.3) % python -m unittest tests.test_discover
...
----------------------------------------------------------------------
Ran 3 tests in 3.615s

OK