Pass unrecognized Content-Encoding values through instead of failing (#2518)

Since 8bba34e, prepare_content_receiver() rejected every non-empty
Content-Encoding that create_decompressor() could not handle, conflating an
unrecognized content coding with a known one whose support was not compiled
in. A response labeled `Content-Encoding: UTF-8` -- a header some servers
misuse to advertise a character set -- therefore failed with 415, which the
client surfaced as the generic Error::Read. Before that commit the raw body
came back untouched.

Restore that behavior: reject only a coding cpp-httplib recognizes but was
not built with, and leave an unrecognized one (including "identity") alone.
Match codings case-insensitively while here, as RFC 9110 8.4.1 requires;
otherwise `Content-Encoding: GZIP` would look unrecognized and hand back a
still-compressed body.

open_stream() had the mirror-image problem. It silently passed the payload
through whenever create_decompressor() returned null, so a gzip response on
a build without zlib reached the caller still compressed, and it never
checked is_valid() -- gzip_decompressor::decompress() only asserts, so a
failed inflateInit2() was undefined behavior in release builds. It now
applies the same policy as the buffered path.

Add Error::UnsupportedContentEncoding so callers can tell this apart from a
read failure, and report an unusable decompressor as Error::Compression. The
status read_content() writes was previously discarded into an uninitialized
dummy_status.

README.md

@@ -936,6 +936,7 @@ enum class Error {
   UnsupportedAddressFamily,
   HTTPParsing,
   InvalidRangeHeader,
+  UnsupportedContentEncoding,
 };
 ```