ShaderUI : Fix user defaults for parameter child plugs
ShaderUI : Fix user defaults for parameter child plugs

Fixes the following test failure :

```
======================================================================
FAIL: testSplineParameterUserDefaults (GafferOSLUITest.OSLShaderUITest.OSLShaderUITest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/john/dev/build/gaffer/python/GafferOSLUITest/OSLShaderUITest.py", line 110, in testSplineParameterUserDefaults
    self.assertEqual( s["parameters"]["spline"]["interpolation"].getValue(), Gaffer.SplineDefinitionInterpolation.Linear )
AssertionError: 1 != Gaffer._Gaffer.SplineDefinitionInterpolation.Linear
```

This appears to have been broken by #2536, which was pull-requested before the test was added, but merged after the test had been merged. Something to bear in mind if we keep PRs open for a long time - the Travis results are only valid at the point they were run, and aren't to be trusted after subsequent changes to master.

johnhaddon/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,boberfly/gaffer,hradec/gaffer,ivanimanishi/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,boberfly/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,boberfly/gaffer,ImageEngine/gaffer,ivanimanishi/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,hradec/gaffer,andrewkaufman/gaffer,hradec/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,hradec/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,boberfly/gaffer,ivanimanishi/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,hradec/gaffer,lucienfostier/gaffer