CDRIVER-4678 Fix $code and $dbPointer array parsing

[joshbsiegel]

Summary

This PR will fix an error when parsing an array with $code or $dbPointer where the implicit keys (0, 1, 2, etc.) would not get correctly set on the values when converting to BSON.

Background

Previously, when parsing JSON such as

 [{"$code" : "A"}]

the BSON outputted would not correctly attach a key of 0 to the value of {"$code" : "A"}. The same issue was caused when attempting to parse JSON with $dbpointer.

Fix

The $code and $dbPointer handlers in bson-json.c were the only two types that called _bson_json_buf_set to save the value of the parent key (e.g. {"key": {"$code" : "A"}}, where we want to save "key" somewhere). These function calls were made by passing through bson->key_buf.buf as const void *from. However, bson->key_buf.buf wasn't set to 0, 1, 2, etc., bson->key was. By passing through bson->key instead and still using bson->key_buf.len as the length, the implicit keys will get set correctly.
Although mixing key with key_buf seems unorthodox, this is a practice done elsewhere in the codebase:

mongo-c-driver/src/libbson/src/bson/bson-json.c

Line 1290 in ac436db

STACK_PUSH_DOC (bson_append_document_begin (STACK_BSON_PARENT,

What's New

• Fixed calls to _bson_json_buf_set by passing through bson->key
• Added tests for $code and $dbPointer arrays with 1 and 2 elements

[kevinAlbs]

LGTM with comment removed.

Although mixing key with key_buf seems unorthodox

I think it is expected to use key to refer to the key string. Consider this call:

      bson->key_buf.len = bson_uint32_to_string (
         STACK_I, &bson->key, (char *) bson->key_buf.buf, 12);

bson_uint32_to_string documents that the buffer may or may not be used. The out pointer is always set to the resulting string:

 size_t
  bson_uint32_to_string (uint32_t value,
                         const char **strptr,
                         char *str,
                         size_t size);

Converts value to a string.

If value is from 0 to 999, it will use a constant string in the data section of the library.

If not, a string will be formatted using str and snprintf().

strptr will always be set. It will either point to str or a constant string. Use this as your key.