[nits] Convert request_content_len to i32 because it would be casted to i32 finally

[ysak-y]

What

Make type of request_content_len property in EspHttpConnection struct from u64 to i32.

Why

Now I'm working on #310 . I noticed the second argument of esp_http_client_open method expects i32 value, not u64. But request_content_len type is u64. I investigated requset_content_len property but it seems there is no reason that request_content_len type should be u64. So I want to modify it to i32 because it would make more easier to implement #310 .

When I remove as _ from https://github.com/esp-rs/esp-idf-svc/blob/master/src/http/client.rs#L251 , then shows the error as below.

      |         esp!(unsafe { esp_http_client_open(self.raw_client, self.request_content_len) })?;
      |                       --------------------                  ^^^^^^^^^^^^^^^^^^^^^^^^ expected `i32`, found `u64`
      |                       |
      |                       arguments to this function are incorrect

How

Replace u64 type with i32 of request_content_len property. Then remove as _ syntax for them because now it has expected type.

[Vollbrecht]

To make the process a bit simpler can you make all changes you want to do, in your initial PR with consecutively commits instead of making PR's for each commit.

[ysak-y]

IMO, PRs should be created for a single purpose. Doing so makes it easier to revert or track PRs. However, I will follow your suggestion if it is better.

[ivmarkov]

This PR is modifying the type of a single private variable. It does not have a purpose on its own, only as part of the other PR. Consolidate your work there please. And also follow my comments on the other PR - for example, I suggested there instead of i64 to do Option<u64>. That might or might not be a good idea. Please think about that, and try to come - in the other PR - with a complete solution implementing chunked encoding for POST/PUT requests.