http/json: fix wrong return type of cJSON_type_to_string

We return c style strings instead of strings of C++ String class.

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>
http/json: be more informative about the json schema mismatch error

Before this patch, we might get error meessage like:
    info: HTTP request throw a schema_mismatch_exc_t with what = Expected object instead got: 2

However, we can tell user more by:
    info: HTTP request throw a schema_mismatch_exc_t with what = Expected 'object' instead got 'number': 2

Signed-off-by: Liu Aleaxander <Aleaxander@gmail.com>
Merge pull request #2263 from Aleaxander/yliu-json-trivial-enhancement

yliu json trivial enhancement