v0.4.1

• release: ngx 0.4.1 (9d2ce0d)
• release: nginx-sys 0.2.1 (89eb277)
• fix(#50): user_agent method returns 'Option<&NgxStr>' and not '&NgxStr' (8706830)
• fix(#41): zlib version update (89e0fcc)
• fix: check user_agent for null (0ca0bc9)
• feat: (2 of 2) Upstream module example (#37) (840d789)
• feat: (1 of 2) Supporting changes for upstream example module. (#36) (8b3a119)
• Revert "ci: use GH cache for .cache folder on MAC OS" (7972ae7)
• docs: updated changelog (74604e2)