[client][LogSearchSystem] Fix a bug record change by PUT doesn't work

Django doesn't parse HTTP request body for PUT... We need to parse body
by ourselves.
[client][LogSearchSystem] Stop to set content type for empty body response

If "application/json" content type is set, jQuery try to parse empty
string and reports "invalid JSON" error.
Merge pull request #785 from project-hatohol/client-log-searcy-system-fix-put-data-handling

[client][LogSearchSystem] Fix a bug record change by PUT doesn't work