Register default function as coalesce alias

[malud]

A more user friendly name for coalesce is default. So we would like to Register and create an alias with default to the same underlying function in eval/lib/coalesce.go.

• Register default next to coalesce and point to the same function.
• Do not remove coalesce as registered function
• Change the documentation and replace coalesce with default.
• Extend the TestFunctions (http_integration_test.go) and related configuration file to use both functions (copy of /coalesce endpoint)