don't panic after writing headers

If the encoding fails, respond panics (https://github.com/matryer/respond/blob/master/respond.go#L33). However, it has already written out headers (https://github.com/matryer/respond/blob/master/respond.go#L28), so it's not possible to recover and display an error page, using e.g. this: https://github.com/dimfeld/httptreemux#panic-handling

Start by reading respond.go around lines 28 and 33, following how headers are written before encoding and how an encoding failure becomes a panic. Check the repository's existing tests and response flow; done means encoding failures no longer panic after headers have been written and the behavior supports the cited panic-handling use case.

Written by the indexing model from the issue text.