Fix three bugs (the second and third found incidentally while investigating the first):

1. Exception thrown when the content-type header is templated, due to this not being eagerly resolvable to a valid mime type.
2. Body compressed in an uncompressible format causes exception to be thrown when a LoggedResponse is created.
3. Body that's compressed in an uncompressible format and therefore can't be templated is still converted to a string, thus corrupting it.