Decode headers using utf-8 only if they are not str

[GianOrtiz]

httpie showed a strange behavior whenever I was using sessions with files uploads using multipart forms, returning the following error during execution:

http: error: AttributeError: 'str' object has no attribute 'decode'

So, I tested with debug:

> http --debug --session=test -f POST pie.dev/post name='John Smith' cv@~/files/data.xml
HTTPie 2.3.0
Requests 2.25.1
Pygments 2.7.4
Python 3.7.3 (default, Jul 25 2020, 13:03:44) 
[GCC 8.3.0]
/usr/bin/python3
Linux 4.19.0-13-amd64

<Environment {'colors': 256,
 'config': {'__meta__': {'about': 'HTTPie configuration file',
                         'help': 'https://httpie.org/docs#config',
                         'httpie': '0.9.8'},
            'default_options': []},
 'config_dir': PosixPath('/home/gian/.httpie'),
 'devnull': <property object at 0x7f8f26114e08>,
 'is_windows': False,
 'log_error': <function Environment.log_error at 0x7f8f26042378>,
 'program_name': 'http',
 'stderr': <_io.TextIOWrapper name='<stderr>' mode='w' encoding='UTF-8'>,
 'stderr_isatty': True,
 'stdin': <_io.TextIOWrapper name='<stdin>' mode='r' encoding='UTF-8'>,
 'stdin_encoding': 'UTF-8',
 'stdin_isatty': True,
 'stdout': <_io.TextIOWrapper name='<stdout>' mode='w' encoding='UTF-8'>,
 'stdout_encoding': 'UTF-8',
 'stdout_isatty': True}>

http: error: AttributeError: 'str' object has no attribute 'decode'

Traceback (most recent call last):
  File "/home/gian/.local/bin/http", line 8, in <module>
    sys.exit(main())
  File "/home/gian/.local/lib/python3.7/site-packages/httpie/__main__.py", line 11, in main
    exit_status = main()
  File "/home/gian/.local/lib/python3.7/site-packages/httpie/core.py", line 81, in main
    env=env,
  File "/home/gian/.local/lib/python3.7/site-packages/httpie/core.py", line 196, in program
    for message in messages:
  File "/home/gian/.local/lib/python3.7/site-packages/httpie/client.py", line 63, in collect_messages
    httpie_session.update_headers(request_kwargs['headers'])
  File "/home/gian/.local/lib/python3.7/site-packages/httpie/sessions.py", line 80, in update_headers
    value = value.decode('utf8')
AttributeError: 'str' object has no attribute 'decode'

After some investigation, I noticed the error was generated from the usage of the header returned from get_multipart_data_and_content_type containing the Content-Type header for the multipart form, which returns an encoder and the header as a string, at collect_messages as a byte in update_headers using the decode method to generate a string from it.

To solve this issue I added a new test and then modified the update_headers function to not decode the header if it is already a string.

[jkbrzt]

@GianOrtiz Thanks for the fix & test case! 🥧