Add newlines to FormatProgress for JSON as well

Commit 060da572d20dfeee4fe02ce6b975a6cb33e50dbe has introduced newlines
to streamformatter to help parse Remote API responses. However,
FormatProgress method was omitted from the list of patched methods,
leaving progress messages in, say, /images/create without newlines.

Signed-off-by: Roma Sokolov <sokolov.r.v@gmail.com>