Comment by CodesInChaos

I think the only sane approach for caching comparing if the body is identical and not apply any content type specific transformations. It's the safest choice, and the cache not being effective in some edge cases isn't a big deal. The client can always canonicalize the data itself, if it matters for its use-case.

Even if the spec says that different representations are equivalent, it's quite common for applications to treat them differently. For example field ordering in json objects is supposed to not matter, but some serializers care if a type discriminator is the first field.