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 does not just seem redundant but can also lead to `505 HTTP Version Not Supported` errors when exceeding the maximum URL length allowed by AWS (apparently 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) and in turn 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
Do I see it correctly that this is the only non-whitespace change?