Do not send a query string when PostQuery is used.

At present, any API operations that use the PostQuery request method send both, a query string and an `application/x-www-form-urlencoded` request body. This is not just redundant but can also lead to "505 HTTP Version Not Supported" errors when exceeding the maximum URL length allowed by AWS (the request line will be truncated and thus leads to misinterpretation of the HTTP version).

I ran into this with `SES` where requests were returning sporadic `505`s (the length of the email bodies varied a bit based on some variables, sometimes exceeding the query string limit) leading `aws` to fail with `Missing root element` in an attempt to parse a non-existing response body.

I'm not completely certain that this doesn't cause issues with some other APIs that I haven't tested, but it looks like an oversight to me that a query string is still being sent when `PostQuery` is used.

Related prior issue in boto: https://github.com/boto/boto/pull/1081