Fix #22862 - change the httpclient user-agent to be valid spec-wise

[ghost]

Per https://datatracker.ietf.org/doc/html/rfc9110#name-user-agent a User-Agent is defined as follows:

  User-Agent = product *( RWS ( product / comment ) )

Where

  product         = token ["/" product-version]
  product-version = token

In this case, token is defined in RFC 7230 - https://datatracker.ietf.org/doc/html/rfc7230#section-3.2.6:

     token          = 1*tchar

     tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
                    / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
                    / DIGIT / ALPHA
                    ; any VCHAR, except delimiters

or, in the original RFC 2616 - https://datatracker.ietf.org/doc/html/rfc2616#section-2.2 (next page):

       token          = 1*<any CHAR except CTLs or separators>
       separators     = "(" | ")" | "<" | ">" | "@"
                      | "," | ";" | ":" | "\" | <">
                      | "/" | "[" | "]" | "?" | "="
                      | "{" | "}" | SP | HT

which means that a token cannot have whitespace. Not sure if this should be in the breaking changelog section - theoretically, some clients might've relied on the old Nim user-agent?

For some extra info, some other languages seem to have adopted the same hyphen user agent to specify the language + module, e.g.:

• https://github.com/python/cpython/blob/main/Lib/urllib/request.py#L1679 (Python-urllib/<version>)

Fixes #22862.

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