Fix deferred_response content parameter not being used (issue #331)

[etr]

Summary

• Fix the deferred_response constructor's content parameter which was documented to send initial content before callback data, but was never actually used
• The content is now stored and sent first before calling the user's callback

Changes

• Store content in initial_content member instead of passing to base class string_response
• Track content_offset for proper chunking when content exceeds buffer size
• Send initial content bytes before invoking user's cycle_callback
• Update existing test expectations to include the initial content
• Add new test for backward compatibility with empty content parameter

Test plan

• Existing tests updated and passing (make check)
• New empty-content backward compatibility test added and passing
• All 12 tests pass