Make inputs with defaults implicitly optional

[mlin]

A call may supply None for an input with a non-optional type if it has a default expression, in which case the default will be used. In contrast, if the input has both a default expression and an optional type, None would be used. #505

[coveralls]

Pull Request Test Coverage Report for Build 917187617

• 13 of 13 (100.0%) changed or added relevant lines in 5 files are covered.
• 20 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.02%) to 96.214%

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Files with Coverage Reduction	New Missed Lines	%
WDL/_parser.py	1	95.58%
WDL/runtime/task_container.py	1	93.1%
WDL/runtime/task.py	1	95.06%
WDL/_util.py	4	90.69%
WDL/runtime/download.py	13	89.26%

Totals
Change from base Build 860189830:	-0.02%
Covered Lines:	6684
Relevant Lines:	6947

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💛 - Coveralls

[mlin]

Punted for now: should inputs JSON also be able to put null for a non-optional input type?