Uniformize UTF-8 naming (#1115)

* Uniformize UTF-8 naming

Replace `utf8` -> `utf-8` everywhere.
It should have no impact, `utf8` is an alias of `utf-8` [1].

[1] ee03bad25e/Lib/encodings/aliases.py (L534)

* Always specify the encoding

Let's be explicit over implicit. And prevent future warnings from PEP-597 [1].

[1] https://www.python.org/dev/peps/pep-0597/#using-the-default-encoding-is-a-common-mistake

* Update `UTF8` constant (`utf-8` -> `utf_8`)

* Remove default argument from `str.encode()` and `bytes.decode()`

* Clean-up

httpie/client.py

@@ -24,7 +24,7 @@ from .utils import get_expired_cookies, repr_dict
 
 urllib3.disable_warnings()
 
-FORM_CONTENT_TYPE = 'application/x-www-form-urlencoded; charset=utf-8'
+FORM_CONTENT_TYPE = 'application/x-www-form-urlencoded; charset=UTF-8'
 JSON_CONTENT_TYPE = 'application/json'
 JSON_ACCEPT = f'{JSON_CONTENT_TYPE}, */*;q=0.5'
 DEFAULT_UA = f'HTTPie/{__version__}'
@@ -188,7 +188,7 @@ def finalize_headers(headers: RequestHeadersDict) -> RequestHeadersDict:
             value = value.strip()
             if isinstance(value, str):
                 # See <https://github.com/httpie/httpie/issues/212>
-                value = value.encode('utf8')
+                value = value.encode()
         final_headers[name] = value
     return final_headers