Fix DefaultBinder empty body handling for unknown ContentLength

Fix issue where POST requests with empty bodies and ContentLength=-1
(unknown/chunked encoding) incorrectly fail with 415 Unsupported Media Type.

The DefaultBinder.BindBody method now properly detects truly empty bodies
when ContentLength=-1 by peeking at the first byte. If no content is found,
it returns early without error. If content exists, it reconstructs the body
to preserve the original data.

Fixes #2813

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

bind.go

@@ -8,6 +8,7 @@ import (
 	"encoding/xml"
 	"errors"
 	"fmt"
+	"io"
 	"mime/multipart"
 	"net/http"
 	"reflect"
@@ -71,6 +72,22 @@ func (b *DefaultBinder) BindBody(c Context, i interface{}) (err error) {
 		return
 	}
 
+	// For unknown ContentLength (-1), check if body is actually empty
+	if req.ContentLength == -1 {
+		// Peek at the first byte to see if there's any content
+		var buf [1]byte
+		n, readErr := req.Body.Read(buf[:])
+		if readErr != nil && readErr != io.EOF {
+			return NewHTTPError(http.StatusBadRequest, readErr.Error()).SetInternal(readErr)
+		}
+		if n == 0 {
+			// Body is empty, return without error
+			return
+		}
+		// There's content, put the byte back by creating a new reader
+		req.Body = io.NopCloser(io.MultiReader(strings.NewReader(string(buf[:n])), req.Body))
+	}
+
 	// mediatype is found like `mime.ParseMediaType()` does it
 	base, _, _ := strings.Cut(req.Header.Get(HeaderContentType), ";")
 	mediatype := strings.TrimSpace(base)