Ignore empty list elements in the Accept header (Fix #2567)

parse_accept_header() rejected any Accept value with a leading, trailing
or doubled comma, and Server::process_request() validates Accept before
routing, so "Accept: text/html," was answered 400 Bad Request on every
route.

RFC 9110 Section 5.6.1.2 requires a recipient to parse and ignore empty
list elements in a #rule list, so those values are legal. split() already
trims each element and skips the empty ones, which made the guard inside
the callback unreachable as well; drop both and let the empty elements
fall away. The header length limit bounds how many a sender can send, so
ignoring all of them cannot be used as a denial-of-service vector.

get_combined_header_value() keeps skipping empty field lines, but that
skip is no longer observable through a request now that a stray comma
parses cleanly, so it gets its own test.

httplib.h

@@ -7858,8 +7858,7 @@ inline std::string get_combined_header_value(const Headers &headers,
   for (auto it = rng.first; it != rng.second; ++it) {
     // RFC 9110 Section 5.6.1.2: a recipient has to parse and ignore empty list
     // elements, so an empty field line must not contribute a bare comma to the
-    // combined value. parse_accept_header() rejects a leading comma outright,
-    // which would turn a legal request into 400 Bad Request.
+    // combined value.
     if (it->second.empty()) { continue; }
     if (!combined.empty()) { combined += ", "; }
     combined += it->second;
@@ -8836,12 +8835,6 @@ inline bool parse_accept_header(const std::string &s,
   // Empty string is considered valid (no preference)
   if (s.empty()) { return true; }
 
-  // Check for invalid patterns: leading/trailing commas or consecutive commas
-  if (s.front() == ',' || s.back() == ',' ||
-      s.find(",,") != std::string::npos) {
-    return false;
-  }
-
   struct AcceptEntry {
     std::string media_type;
     double quality;
@@ -8852,16 +8845,16 @@ inline bool parse_accept_header(const std::string &s,
   int order = 0;
   bool has_invalid_entry = false;
 
-  // Split by comma and parse each entry
+  // Split by comma and parse each entry. RFC 9110 Section 5.6.1.2: a recipient
+  // has to parse and ignore empty list elements, so a leading, trailing or
+  // doubled comma must not turn a legal Accept value into 400 Bad Request.
+  // split() skips them, and the header length limit bounds how many a sender
+  // can send, so ignoring all of them cannot be used as a denial-of-service
+  // vector.
   split(s.data(), s.data() + s.size(), ',', [&](const char *b, const char *e) {
     std::string entry(b, e);
     entry = trim_copy(entry);
 
-    if (entry.empty()) {
-      has_invalid_entry = true;
-      return;
-    }
-
     AcceptEntry accept_entry;
     accept_entry.order = order++;