Fix #22862 - change the httpclient user-agent to be valid spec-wise (#22882)

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.

changelog.md

@@ -4,6 +4,7 @@
 ## Changes affecting backward compatibility
 
 - `-d:nimStrictDelete` becomes the default. An index error is produced when the index passed to `system.delete` was out of bounds. Use `-d:nimAuditDelete` to mimic the old behavior for backwards compatibility.
+- The default user-agent in `std/httpclient` has been changed to `Nim-httpclient/<version>` instead of `Nim httpclient/<version>` which was incorrect according to the HTTP spec.
 
 ## Standard library additions and changes
 

lib/pure/httpclient.nim

@@ -359,7 +359,7 @@ type
                                         ## and `postContent` proc,
                                         ## when the server returns an error
 
-const defUserAgent* = "Nim httpclient/" & NimVersion
+const defUserAgent* = "Nim-httpclient/" & NimVersion
 
 proc httpError(msg: string) =
   var e: ref ProtocolError