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).

Related prior issue in boto: https://github.com/boto/boto/pull/1081
Merge pull request #145 from romanb/bugfix/post-query

Do not send a query string when PostQuery is used.