Appendix on converting data types to strings (3.0.4)

[handrews]

Rendering with all currently open 3.0.4 changes (as of the timestamp of this edit - use the dropdown on the word "edited" in the bar for this comment) can be found here.

Fixes:

• Behaviour of pattern (regex) with url encoding #2045
• Can we get clarification on how to post multipart/form-data containing an array of items of type X? #2840 (comment) (but not the parts about interactions with explode which are 3.1-specific and will be addressed separately)
• (De-)Serializing null, required, and empty values ins OAS parameters #2037 (perhaps not ideally, but I think the best we can do without inventing a new standard)

This is an appendix because it's pretty much just advice for things that otherwise feel like an erroneous gap in the spec - but as with PR #3818, this is stuff where it's hard to know where to look or what to ask, so having it in the spec is important.

It's very unclear how numbers, booleans, and other non-UTF-8-string values are converted to strings, particularly for the form media types. This adds a brief appendix that acknowledges the lack of standardization, and points to resources for the few cases that do have specifications.

It highlights concerns with relying on certain JSON Schema keywords or values for serialization, and suggests defining schemas of type string and requiring applications to perform the conversion prior to schema validation as a way to control the results.

This also clarifies that schema validation occurs before serialization.

QUESTION: I was pretty casual about where I put the "See Appendix C" notes - are there better places?

[ralfhandl]

+1, two minor nits

[handrews]

The most recent force-push was just to resolve merge conflicts. There were previously two TSC approvals, so this should get merged ASAP and only needs one re-approval.