Modifications of headers returned by Client.prepare_*_request affects future calls

Describe the bug

If I modify the headers dictionary returned by a Client.prepare_*_request method, the future calls to any of these methods return the modified headers.

How to reproduce

>>> import oauthlib.oauth2
>>> client = oauthlib.oauth2.WebApplicationClient('my client id')
>>> _, headers, _ = client.prepare_authorization_request('https://example.com')
>>> headers
{'Content-Type': 'application/x-www-form-urlencoded'}
>>> headers['Content-Length'] = '0'
>>> _, headers, _ = client.prepare_authorization_request('https://example.com')
>>> headers
{'Content-Type': 'application/x-www-form-urlencoded', 'Content-Length': '0'}
>>> _, headers, _ = client.prepare_token_request('https://example.com')
>>> headers
{'Content-Type': 'application/x-www-form-urlencoded', 'Content-Length': '0'}
>>> _, headers, _ = client.prepare_refresh_token_request('https://example.com')
>>> headers
{'Content-Type': 'application/x-www-form-urlencoded', 'Content-Length': '0'}

Expected behavior

Either:

>>> import oauthlib.oauth2
>>> client = oauthlib.oauth2.WebApplicationClient('my client id')
>>> _, headers, _ = client.prepare_authorization_request('https://example.com')
>>> headers['Content-Length'] = '0'
>>> _, headers, _ = client.prepare_authorization_request('https://example.com')
>>> headers
{'Content-Type': 'application/x-www-form-urlencoded'}
>>> _, headers, _ = client.prepare_token_request('https://example.com')
>>> headers
{'Content-Type': 'application/x-www-form-urlencoded'}
>>> _, headers, _ = client.prepare_refresh_token_request('https://example.com')
>>> headers
{'Content-Type': 'application/x-www-form-urlencoded'}

or

>>> import oauthlib.oauth2
>>> client = oauthlib.oauth2.WebApplicationClient('my client id')
>>> _, headers, _ = client.prepare_authorization_request('https://example.com')
>>> try:
...     headers['Content-Length'] = '0'
... except Exception:
...     print('an exception occurred')
... 
an exception occurred

or

this unusual feature being documented

Additional context

Please provide any further context here.

• Are you using OAuth1, OAuth2 or OIDC? OAuth2
• Are you writing client or server side code? client
• If client, what provider are you connecting to? a custom one
• Are you using a downstream library, such as requests-oauthlib, django-oauth-toolkit, ...? no

Start with the Client.prepare_authorization_request, prepare_token_request, and prepare_refresh_token_request entry points shown in the reproduction. Reproduce the shared-header behavior, then trace how each method creates or reuses its returned headers. Done means modifying one returned headers dictionary does not affect later calls, with regression coverage for the demonstrated OAuth2 client behavior.