Have issue handle literal value $50 in mapping

[lixingwang]

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[] enhancement request
[] feature request
[] support request
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Current behavior (how does the issue manifest):

Expected behavior: Take $50 as a literal value, not a mapping ref Minimal steps to reproduce the problem (not required if feature enhancement):

What is the motivation / use case for changing the behavior?

Please tell us about your environment (Operating system, docker version, browser & verison if webui, etc):

Flogo version (CLI & contrib/lib. If unknown, leave empty or state unknown): 0.X.X

Additional information you deem important (e.g. issue happens only occasionally):