Wrong OptArg serialisation after OrderBy

### Version Information
| .NET Full Framework? | 4.8 |
| RethinkDB Server | 2.3.6 |

Hi I have a very simple indexed orderby:
```
R.Db("db").Table("table").OrderBy().OptArg(R.Desc("lastName")).Limit(100)
```

This gives me:
```
RethinkDb.Driver.ReqlServerCompileError
Expected a TermType as a NUMBER but found STRING.
```
It produces this raw string:
```
$reql_reqlraw$[71,[[41,[[15,[[14,["db"]],"table"]]],{"index":["DESC",["lastName"]]}],100]]
```
Notice the "DESC" in the raw string? Should be 74. I believe OptArgs have a serialisation bug where they use globally defined JsonConvert.Settings accidentally and therefore the enum is serialised as string.

Any workarounds?

Reproduce the indexed OrderBy query shown in the issue and inspect OptArg serialization, especially the global JsonConvert.Settings implicated by the report. Done when the generated raw ReQL uses numeric term type 74 instead of the string "DESC" and the query compiles.

Written by the indexing model from the issue text.