Implement content negotiation for Accept header

[yawkat]

This patch adds support for proper content negotiation using the Accept header. This includes new support for:

• "Partial" matches, like text/*
• Quality parameters
• Correct matching by precedence, e.g. matching text/plain over */*.

Compatibility considerations:

• MediaType now considers parameters in matches, if specified
• Overloaded routes that relied on the old, incorrect matching behavior will now match differently.

Resolves #6637