[macOS] Fix curl error "argument list too long"

[HLFH]

Hi,

The WebClient.cls operates badly for curl.

When the request body is too large, the PostJson and Execute methods are triggering the error:

ERROR - WebClient.Execute: -2147210493 (11011 / 80042b03), An error occurred during execute
-2147210494 (80042b02): An unknown cURL error occured, #127
Find details at http://curl.haxx.se/libcurl/c/libcurl-errors.html

Error #127 means the curl command has failed. The Terminal will display the following error:

Argument list too long

There is a maximum length of arguments for a new process.

I am running VBA-Web on macOS 10.14.4.

hlfh$ getconf ARG_MAX
262144

The maximum length of arguments for the curl process on the current macOS is 262144 bytes which is 0.262 megabyte and the length of my curl -d argument was 0.544 megabyte.

Therefore, I will submit a PR to this repo to fix the WebClient.cls file on his function PrepareCurlRequest.

Thanks.

[HLFH]

Done on #397