INT-3054 URI Variable Mapping Improvements

Permit missing URI vars in HTTP outbound gateway

Do not attempt to evaluate uri variables that are not in the URL

The http gateway might not know what uri variables it might obtain in the URI.
This patch makes sure that only the uri variables defined in the uri are parsed (the rest are ignored)
INT-3054: Add ExpressionEvalMap Support

Use an immutable Map wrapper to evaluate expressions
only when accessed.

Polishing
Merge pull request #822 from wwadge/INT-3054

* INT-3054:
  INT-3054: Add ExpressionEvalMap Support
  INT-3054 URI Variable Mapping Improvements