Encode the characters RFC 3986 doesn't allow in a client path

encode_path() sent '"', '<', '>', '\', '^', '`', '{', '|', '}', '['
and ']' raw, although RFC 3986 §3.3 doesn't allow them in a path.
Tomcat, Jetty and Undertow answer each of them with 400 by default, but
accept the percent-encoded form. It also stopped at an embedded NUL, so
"/a\0b" went out as "/a". Encode every byte outside pchar, keeping '/'
and '%' so that escapes pass through, and still encode '+', ',', ';'
and '\'' as before.

encode_request_target() now passes only the path to encode_path(). The
"http://host" of a target sent to a proxy and a CONNECT "host:port" are
sent as they are, so the brackets of an IPv6 host stay.

README.md

@@ -1471,7 +1471,7 @@ auto res = cli.Get("/already%20encoded/path"); // Use pre-encoded paths
 ```
 
 - `set_path_encode(bool on)` - Controls automatic encoding of special characters in URL paths
-  - `true` (default): Automatically encodes spaces, plus signs, newlines, and other special characters
+  - `true` (default): Percent-encodes characters not allowed in a URL path (spaces, control characters, `<`, `|`, non-ASCII bytes, etc.) and `+`, `,`, `;`, `'`. `%` is kept, so existing escapes aren't encoded twice
   - `false`: Sends paths as-is without encoding (useful for pre-encoded URLs)
 
 ### Performance Note for Local Connections