Fix issue #519

[tjanc]

Context

1. A fixedType Array element should be treated as the type of lists of any of the elements in its content.
2. An Enum element should be treated as the type of any of the elements in its enumerations attribute.

One could argue both any of combinators are expected to behave similarly.

Current behaviour

In (1) the JSON value rendering precedence is content - sample - default.
In (2) the JSON value rendering precedence is content - sample - default - enumerations.

This is unified only syntactically. In (1) what seems to be content actually is a fixedType'd Array element's way to define its enumerations.

Fix

Issue #519 comes with this suggestion:
In (1) the JSON value rendering precedence SHOULD BE sample - default - content.

This PR does just this, it changes the precedence of value source for fixedType Array elements in the JSON value renderer.

[pksunkara]

The problem is not about the fixed-type. It's about the sample described in array taking precedence. The #519 issue should produce the same expected JSON value even with no fixed-type flag there.

[kylef]

Discussed with @tjanc, we will close this PR as we should find a way to solve this more generically instead of a work-around on fixed-type specifically. Closing but leaving branch for future reference.

[kylef]

We'll likely have to change MSON / API Elements to be able to solve this problem.