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_test.go

@@ -1082,6 +1082,14 @@ func TestDefaultBinder_BindBody(t *testing.T) {
 			givenContent:     strings.NewReader(""),
 			expect:           &Node{ID: 0, Node: ""},
 		},
+		{
+			name:             "ok, POST with empty body and ContentLength -1 (Issue #2813)",
+			givenURL:         "/api/real_node/endpoint?node=xxx",
+			givenMethod:      http.MethodPost,
+			givenContent:     strings.NewReader(""),
+			whenChunkedBody:  true, // This sets ContentLength to -1
+			expect:           &Node{ID: 0, Node: ""},
+		},
 		{
 			name:             "ok, JSON POST bind to struct with: path + query + chunked body",
 			givenURL:         "/api/real_node/endpoint?node=xxx",