v0.18.27: Fix handling of 'NaN' in MetadataDecoder

Fixes an issue where NaN was allowed as a value for Float and Double metadata fields, but then caused an exception when handling smithy.api#range constraints.

What's Changed

• Handle edge cases with NaN/Infinity by @lewisjkl in #1624

Full Changelog: v0.18.26...v0.18.27