httpc: encode body in http request

client_object:request() could pass a body as a string value to HTTP
request when body is used. User must encode body to a string value as a
preparation step. However, we can do encoding for user
automatically when it possible.

@TarantoolBot document
Title: Document body encoding in http request

Now user could pass a "body" option as a value with standard Lua types
(table, nil, string, userdata, cdata, boolean or number) as well as
Tarantool's own data types like decimal, uuid, datetime. In such case
that value will be encoded to a string automatically where it possible.

Encoding depends on body's data type: `nil` converted to an empty
string, types (`string`, `number`, `boolean` and `nil`) will be
converted to a string using standard Lua function `tostring()`, types
cdata, userdata and tables will be encoded by functions defined in
`http.encoders` or `http.new().encoders` table. In latter case encoding
function depends on a content type passed to HTTP request using HTTP
header. Default content type "application/json" and appropriate encoder
are used when content type is missed in HTTP request.

By default encoders for the following content types are defined:

- "application/json"
- "application/msgpack"
- "application/yaml"

For content types missed in `http.encoders` user could define it's own
encoder by defining a new record with a key equal to desired MIME type
in lowercase and it's encoding function that must accept HTTP body and
content type and must return a string with serialized data:

```
local http = require("http")
local xml = require("luarapidxml")

http.encoders = {
    ['application/xml'] = function(body, _content_type)
        return xml.encode(body)
    end,
}
```

Be careful with defining your own encoding function - header with content
type could contain content type like "text/html" as well as a header
options [1], like "text/html; charset=UTF-8". Content type is passed to
encoder function *with* options.

JSON encoder will use default configuration settings defined in JSON
module (see `json.cfg()`). Header with content type will be set to
"application/json" if it was not defined by user in request's options.

```
local http = require("http")
local client = http.new()

local body = {
    a = 1,
    b = 2,
    c = 3,
}
client:request("POST", uri, body)
client:post(uri, body)
```

1. https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Type

Part of #6833