Avoid converting an IP address in headers to a symbol.

I saw if there's a value which matches `*scanner-numeric*`, http-parse converts it into a number by `read-from-string`.
However, the problem is that `*scanner-numeric*` also matches to an IP address without a port number, which means it will be converted into a "_SYMBOL_", not a string.

Here's an example of "Host" header.

```common-lisp
(http-parse::convert-headers-plist (format nil "User-Agent: httperf/0.9.0~C~CHost: 127.0.0.1:8080" #\Return #\Newline))
;=> (:USER-AGENT "httperf/0.9.0" :HOST "127.0.0.1:8080")

(type-of (getf * :host))
;=> (SIMPLE-ARRAY CHARACTER (14))
```

```common-lisp
(http-parse::convert-headers-plist (format nil "User-Agent: httperf/0.9.0~C~CHost: 127.0.0.1" #\Return #\Newline))
;=> (:USER-AGENT "httperf/0.9.0" :HOST |127.0.0.1|)

(type-of (getf * :host))
;=> SYMBOL
```

Guessing this is not intended, I'm sending this small patch.
Needless to say, but don't hesitate to reject this if you have a better solution.