Support complex context in Duktape by using eval

[maisumakun]

Just like ExecJS::ExternalRuntime::Context#call, not using Duktape's #call_prop but simple eval will solve #45 .

I don't know if there might be any side effects for this change, but it seems working.

[rails-bot]

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[judofyr]

@rafaelfranca Any progress on this?

[rafaelfranca]

It is missing tests, so it is hard to assert it is actually doing what it should

[judofyr]

I opened #76 to start a proper discussion around how to handle this issue