Use content-type header if present to detect format

Detect request format (HTML or JSON) from `content-type` header.

Previous:
- path extension (.json, .html)
- "Accept" header
- default (.UNKNOWN => .HTML)

Current:
- path extension (.json, .html)
- "Accept" header
- "Content-Type" header
- default (.UNKNOWN => .HTML)

Adjust `http.Headers` to do case-insensitive matching to conform to HTTP
spec: https://www.rfc-editor.org/rfc/rfc9110.html#name-field-names

src/jetzig/http/Headers.zig

@@ -18,10 +18,14 @@ pub fn deinit(self: *Self) void {
     self.headers.deinit(self.allocator);
 }
 
-// Gets the first value for a given header identified by `name`.
+// Gets the first value for a given header identified by `name`. Case-insensitive string comparison.
 pub fn getFirstValue(self: *Self, name: []const u8) ?[]const u8 {
     for (self.headers.items) |header| {
-        if (std.mem.eql(u8, header.name, name)) return header.value;
+        if (name.len != header.name.len) continue;
+        for (name, header.name) |expected, actual| {
+            if (std.ascii.toLower(expected) != std.ascii.toLower(actual)) continue;
+        }
+        return header.value;
     }
     return null;
 }
@@ -78,6 +82,14 @@ test "append" {
     try std.testing.expectEqualStrings(headers.getFirstValue("foo").?, "bar");
 }
 
+test "case-insensitive matching" {
+    const allocator = std.testing.allocator;
+    var headers = Self.init(allocator);
+    defer headers.deinit();
+    try headers.append("Content-Type", "bar");
+    try std.testing.expectEqualStrings(headers.getFirstValue("content-type").?, "bar");
+}
+
 test "iterator" {
     const allocator = std.testing.allocator;
     var headers = Self.init(allocator);

src/jetzig/http/Request.zig

@@ -151,7 +151,10 @@ pub fn render(self: *Self, status_code: jetzig.http.status_codes.StatusCode) jet
 }
 
 pub fn requestFormat(self: *Self) jetzig.http.Request.Format {
-    return self.extensionFormat() orelse self.acceptHeaderFormat() orelse .UNKNOWN;
+    return self.extensionFormat() orelse
+        self.acceptHeaderFormat() orelse
+        self.contentTypeHeaderFormat() orelse
+        .UNKNOWN;
 }
 
 pub fn getHeader(self: *Self, key: []const u8) ?[]const u8 {
@@ -233,6 +236,17 @@ pub fn acceptHeaderFormat(self: *Self) ?jetzig.http.Request.Format {
     return null;
 }
 
+pub fn contentTypeHeaderFormat(self: *Self) ?jetzig.http.Request.Format {
+    const acceptHeader = self.getHeader("content-type");
+
+    if (acceptHeader) |item| {
+        if (std.mem.eql(u8, item, "text/html")) return .HTML;
+        if (std.mem.eql(u8, item, "application/json")) return .JSON;
+    }
+
+    return null;
+}
+
 pub fn hash(self: *Self) ![]const u8 {
     return try std.fmt.allocPrint(
         self.allocator,