DefaultTypeAdapter: Add support for missing custom scalars

[alvaroaleman]

The default type adapter already supports some custom scalar types but
not all, this change adds the missing ones. The most only used one is likely string.

Fixes #884

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[TristonianJones]

/gcbrun

[TristonianJones]

/gcbrun

[TristonianJones]

/gcbrun